RIGA, Latvia - As the United States and NATO inject personnel and equipment into Eastern Europe in response to Russia's invasion of Ukraine, vulnerable allies such as Latvia are scrambling to scale up their defenses for fear they will be next to come under attack.

Like Ukraine, which is not a NATO member but considered a close partner of the alliance, the countries closest to Russia say they are desperate for more Western military aid. It is essential to arm themselves as well as Ukraine, Latvian Defense Minister Artis Pabriks said during a visit from Defense Secretary Lloyd Austin this month, because there is a real risk the war will "come to our borders."

The Biden administration has vowed to boost side-by-side exercises in the region to hone proficiency in air-defense capability and other vital combat skills, not only in Latvia but across the Baltics and in other nations within easy striking distance of Russian forces. About 100,000 U.S. troops are deployed across Europe, an increase of 20,000 in recent months, with a growing center of gravity in the east. But for those on Russia's doorstep, it's not yet enough.

The NATO members bordering Russia and Belarus - which, once considered a buffer state, has functioned as a forward-operating base for Russian troops since the start of the Ukraine war - are pleased, they say, that the United States along with Europe's financial powerhouses have embraced the view that Russia poses an existential threat to the West.

The military investments made over the past six months are accepted with gratitude, but leaders in the region believe the alliance must become more aggressive in the long term. They are mindful of the resistance from some corners of Congress to moving more U.S. personnel to Europe during a time of rising tensions with China, but most insist that having a greater American footprint in Europe is necessary to keeping Moscow at bay.

Even more vital, Baltic and Eastern European officials say, is a turbocharging of defense production lines to accelerate fulfillment of long-standing orders for weapons that these front-line countries say they require.

"HIMARS, Reapers, counter-battery radars: these are what we will need most in terms of military lethal power that is imminently needed to deter Russia," Kusti Salm, secretary general of the Estonian Defense Ministry, said in an interview. He was referring to high-mobility artillery rocket systems, drones capable of conducting surveillance and precision strikes, and technology used to detect incoming fire.

"We are on the brink of taking risks," Salm said. "Very heavy risks of our own national security tapping into some of our reserves. . . . And I know that there are other allies doing the same. So the only solution is rapidly ramping up the manufacturing power, and making sure the policy framework and policy financing signal support for this."

Earlier this year, Congress approved hundreds of millions of dollars to support, train and equip foreign countries that aided the Ukraine war effort - a category that includes all of the frontline NATO states - as part of a $40 billion package of assistance for the government in Kyiv and other measures to strengthen Western defenses.

Part of the initiative calls for accelerating efforts to replace with NATO-standard weapons the Soviet-legacy systems many frontline states rushed to Ukraine early in the conflict. Many of those countries also have been supplying Ukraine with NATO-compatible heavy weapons from their own stocks.

A senior U.S. defense official, speaking on the condition of anonymity under ground rules set by the Pentagon, said that, in the case of Latvia, its government felt comfortable providing Ukraine with some weapons because Russian forces that had been positioned near their shared border were withdrawn to join the war effort.

But Moscow's hostility toward NATO remains unchanged, this official said, and Western countries need to backfill those capabilities at some point. The official did not say how long that may take. The United States is looking for the Baltic countries to build training ranges and other military infrastructure, the official added, while noting that U.S. security assistance for the region has risen to $180 million this year.

In an interview, the Polish defense attache, Brig. Gen. Krzysztof Nolbert, said "winning that war is absolutely fundamental to security in Europe." Poland is the third-largest donor to the Ukrainian military, Nolbert added, and has routinely urged the West to support Kyiv "more decisively as opposed to incrementally," including by sending in fighter jets.

At the same time, Polish officials believe it would greatly help their own defense posture if the United States could accelerate delivery of certain weapons Warsaw has already been promised. Poland is awaiting Patriot missile batteries, HIMARS, F-16 fighter aircraft and Abrams battle tanks, all along multiyear schedules set before Russia's invasion of Ukraine.

Recently, the head of Poland's national security bureau, Pawel Soloch, spoke with President Joe Biden's national security adviser, Jake Sullivan, about the need for defense production to be more responsive to escalating threats, impressing upon him that the U.S. foreign military financing protocols needed an upgrade, according to people familiar with the conversation who spoke on the condition of anonymity to detail a private conversation.

The White House did not immediately respond to a request for comment about the characterization of their discussion.

"I know they are working at maximum speed," Nolbert said. But, he added: "It's an emergency situation. We need it now."

In Latvia, meanwhile, the defense minister, Pabriks, told reporters this month that his country is seeking the sophisticated long-range rocket artillery that has bedeviled Russian troops in eastern Ukraine plus air and coastal defense systems generally out of reach for countries with modest budgets.

Latvia now considers the Belarus and Russian borders as one and the same, Pabriks said, and officials here closely watch what's happening on the other side with the aid of intelligence provided by the United States and other partners.

There are currently about 600 Americans deployed in Latvia, up from about 100 last winter.

The persistent deployments of NATO troops and weapons to countries along Russia's flank is a strategy Western military leaders call the porcupine defense. It seeks to make the idea of invasion unpalatable to adversarial war planners by demonstrating NATO troops can instantly mobilize and back up allies already toughened by Western training and equipment.

During a stop at Latvia's Lielvarde Air Base, where U.S. troops have taken up residence, Austin heard from a Latvian service member who professed to be the first in his country to return from Black Hawk helicopter training in the United States. In recent years, Latvian pilots were more focused on missions like search and rescue, he told the defense secretary, but now he knows how to fly in combat.

The meeting underscored, though, that even some small challenges remain. One U.S. soldier, deployed here from Ohio, disclosed that it's often difficult to work with his Latvian counterparts in person. They're based an hour's drive from one another, the soldier said, and transportation is lacking.

"We'll work on that," Austin responded. "We'll figure it out. . . . We're going to make sure we know the people that we're committed to fighting alongside, and you get a chance to see some of the land you may have to protect one day."

Pabriks sounded a more urgent and ominous tone, telling the assembled troops, "If something happens on our borders, we are ready to die."

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Demirjian reported from Washington. The Washington Post's Michael Birnbaum in Warsaw contributed to this report.

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