SAN ANTONIO — National Guard troops under Texas Gov. Greg Abbott’s command are blocking U.S. Border Patrol agents from a stretch of the Rio Grande, the Biden administration said in an emergency appeal Friday, heightening tensions over how to handle a record migrant influx.

Texas Department of Public Safety and National Guard officials began erecting fencing and razor wire around the riverfront Shelby Park on Wednesday in Eagle Pass, the border crossing where thousands of migrants have arrived in recent months.

“Texas has the legal authority to control ingress and egress into any geographic location in the state,” Abbott told reporters Friday. That power, he said, is being asserted in Eagle Pass “to maintain operational control.”

The move prompted the Biden administration to ask the Supreme Court to quickly intervene. Government lawyers said in a court filing Friday that Texas cannot control Border Patrol’s access to the river or dictate their duties. The state’s border with Mexico along the Rio Grande is an international boundary under the jurisdiction of federal authorities.

The takeover marks yet another escalation of Abbott’s years-long conflict with federal immigration officials over control of the southwest border in Texas and raises the constitutional question over their role in stanching the flow of migrants entering the state between ports of entry.

“He’s picking a fight,” Steve Vladeck, a constitutional law expert at the University of Texas School of Law, said. “If he loses, he can say that he tried.”

In December, the U.S. Court of Appeals for the 5th Circuit of Appeals temporarily blocked federal agents from cutting or moving razor wire Texas placed on the Rio Grande riverbank that the federal government argues interferes with their work.

The Biden administration told the court Friday that Abbott’s latest move is significant because it interferes with Border Patrol’s “ability to patrol or even to surveil the border and be in a position to respond to emergencies,” according to the filing.

Border Patrol agents used the riverfront park to set up a temporary outdoor holding area when thousands of migrants crossed illegally during a record influx in the Eagle Pass area last month.

The area is the site of a key railway crossing between Mexico and the United States, and large groups of migrants have reached the U.S. border by riding freight trains. After wading across the Rio Grande, they typically surrender to U.S. border agents in hopes they will be released while their claims for humanitarian protection are pending in U.S. immigration court.

Any migrants apprehended by Texas state police or National Guard troops will have to be delivered to U.S. Border Patrol agents because the state does not have the ability to carry out deportations on its own, said a senior CBP official tracking the standoff. The official, who was not authorized to speak to reporters, said it was the first time they have seen Border Patrol’s access to publicly-owned land blocked.

Abbott’s forces have cut off federal agents’ access to the area with concertina wire and blocked roadways with their Humvees, according to the court filing. Federal agents are only allowed to enter the area in case of medical emergencies, officials said.

U.S. Customs and Border Protection officials declined to comment on Abbott’s action, referring inquiries to the emergency court filing.

Some legal scholars and U.S. officials said they were not aware of another recent instance of a state blocking the federal government’s access to land where it has jurisdiction. The Border Patrol has broad authorities to investigate illegal activity along the border, including the ability to access private land without a warrant within 25 miles of an international boundary.

Ilya Somin, a law professor at George Mason University, noted there have been instances of states declining to assist the federal government with immigration enforcement, such as California’s “sanctuary” policies that limit cooperation with U.S. Immigration and Customs Enforcement.

But he said the state cannot block federal authorities and warned that Abbott’s move increases the risk of an armed standoff.

“We’re talking about an international border, and federal law enforcement is supposed to have access to it — certainly on public land,” Somin said. “This creates a potentially dangerous situation where the Border Patrol is facing off against armed National Guard troops, and if someone does something stupid, there could be an unfortunate confrontation.”

Angelo Fernández Hernández, a White House spokesman, accused Abbott of utilizing “extreme political stunts” that make it harder for Border Patrol agents to do their jobs.

“Whether it is leaving migrants on the side of the road in the dead of winter, installing razor wire to make Border Patrol’s job more dangerous, promoting extreme and unconstitutional laws like S.B. 4, or his latest actions in Eagle Pass, Governor Abbott has repeatedly proved that he is not interested in solutions and only seeks to politicize the border,” Fernández Hernández said in a statement.

S.B. 4, a new state law backed by Abbott that expands Texas authorities’ role in immigration enforcement, is also being challenged in federal court by the Biden administration.

Eagle Pass resident Jesse Fuentes, who owns a canoe and kayaking business on the Rio Grande, said he is worried things are going to get ugly in his stretch of the border. He, along with the Biden administration, sued the state when officials installed orange buoys to prevent crossings.

“I’m afraid of what’s coming next,” said Fuentes, who operates from the Shelby Park boat ramp when he has access. “They’ve been pushing each other. Where is it all headed?”

Immigrant advocates responded with alarm when Abbott said last week that “the only thing that we’re not doing is we’re not shooting people who come across the border, because of course, the Biden administration would charge us with murder.”

The governor said Friday he was not advocating for the murder of migrants. Abbott said that he was asked a question about where the line was for actions taken by Texas at the border and he pointed out all the legal things the state could do and what would be illegal.

Eagle Pass Mayor Rolando Salinas, who has been critical of the Biden administration’s border policies, said he was surprised to learn the governor would be using emergency powers from a 2021 declaration that helped launch his border security initiative, Operation Lone Star, to seize the park.

For nearly two years, state law enforcement and troops have been patrolling the city’s riverfront property and erecting barriers to deter migrants. In May, Salinas signed an agreement authorizing the state to pursue trespassing charges against any migrants they apprehended on public property on the city’s behalf. But the city is now exploring legal options against the state’s occupation.

“This is not something the city asked for,” Salinas, a Democrat, said in a news conference Thursday. “The city is not in agreement with Border Patrol being kept out.”

Shelby Park is one of the few sources of recreation and economic activity for the residents of the economically depressed border community. The local high school cross-country team can no longer use the trails, and elderly dogwalkers are shooed away by heavily armed troops.

Amerika Garcia Grewal, a volunteer who has helped organize border vigils remembering migrants who died in their crossings, said she is puzzled over the state’s timing because the number of migrants crossing in recent days has declined.

“There’s nobody there now,” she said. “The state of Texas is hurting Eagle Pass.”

Isaak Ruiz, a county employee and self-described citizen journalist, has advocated for local officials to do more and said he supported the governor’s takeover.

“It’s a civil war between the state and feds, and unfortunately Eagle Pass is caught in the middle,” he said. “I’ve never seen my town like this and it’s upsetting.”

Southern Methodist University political scientist Cal Jillson said Abbott’s actions fall in line with his long-held political ambitions to reassert states’ rights and further limit the power of the federal government. In 2016, the governor called for a new constitutional convention to make several amendments to the U.S. Constitution, including restricting the federal government’s ability to override state laws. As Texas attorney general, Abbott spent much of his tenure taking the federal government to court over health care, environmental and immigration policies.

In the border crisis, Jillson said, Abbott has found his vehicle for pushing the issue again by questioning who has the ultimate authority to protect and manage the border.

“He wants to re-offer that question, hoping this more conservative court will come back with a different ruling,” Jillson said.

But there is more to Abbott’s actions, scholars said. The governor and other Republicans are making an argument for states’ rights not unlike the one the Confederacy made in the run up to the Civil War — arguing states have a legitimate right to evaluate federal conduct.

Several legal experts characterized Abbott’s latest action as especially startling.

“The specter of our Border Patrol agents in a standoff with armed members of the Texas National Guard should scare the bejesus out of us,” Vladeck said. “I think it’s not an exaggeration that this is as direct a confrontation between a state and the federal government as we’ve seen since desegregation.”

Ann Marimow contributed to this report.