BEIRUT — The Syrian military rushed reinforcements to the northwest and launched airstrikes Sunday in an attempt to push back insurgents who seized the country’s largest city of Aleppo, as Iran pledged to help the government counter the surprise offensive.

Iran has been a key political and military ally of Syrian President Bashar Assad in the country’s long-running civil war, but it was unclear how Tehran would support Damascus in this latest flareup that began Wednesday. Insurgents led by jihadi group Hayat Tahrir al-Sham launched a two-pronged attack on Aleppo and the countryside around Idlib, before moving toward neighboring Hama province.

On Sunday, government troops created a “strong defensive line” in northern Hama, according to Britain-based opposition war monitor the Syrian Observatory for Human Rights, as they attempted to stall the insurgents’ momentum. Meanwhile, jets pounded the cities of Idlib and Aleppo, killing at least 15 people, according to a group that operates in opposition-held areas.

The surge in fighting has raised the prospect of another violent, destabilizing front reopening in the Middle East at a time when Israel is fighting Hamas in Gaza and Hezbollah in Lebanon, conflicts that have repeatedly threatened to ignite an even wider regional war. It also risks drawing Russia and Turkey — each with its own interests to protect in Syria — into direct heavy fighting against each other.

The insurgents announced their offensive Wednesday, just as a ceasefire between the Lebanese Hezbollah militant group and Israel began, raising some hope that tensions in the region might be calming.

The surprise offensive is a huge embarrassment for Assad, and it comes at a time when his allies — Iran and groups it backs and Russia — are preoccupied with their own conflicts.

According to a statement from Assad’s office, Iranian Foreign Minister Abbas Araghchi said that Tehran was ready to support Damascus in their counteroffensive against the insurgency.

Arab leaders, including Jordan’s King Abdullah II and United Arab Emirates President Sheikh Mohammed bin Zayed Al Nahyan, have also called Assad to express their solidarity.

Opposition troops advance

The insurgents took over most of Aleppo on Saturday and made gains in the surrounding province. They also took control of the main water pumping station for city and it is no longer working, Syrian Minister of Water Resources Moataz Qattan told the pro-government radio station Sham FM.

Elsewhere, rebel commander Col. Hassan Abdulghani said the insurgents advanced in the countryside around Idlib, putting all of the province of the same name under their control.

They also claimed to have entered the city of Hama, but there was no independent confirmation of that.

In Khan Sheikhoun in Idlib province, military vehicles abandoned by Syrian troops dotted the roads. People posed and took pictures of themselves atop one abandoned tank on a highway, while the insurgents grabbed munitions and shells from them before continuing their push deeper into Syria.

The insurgents vowed to push all the way into Damascus, but life in the Syrian capital remained normal with no signs of panic. In southeastern Aleppo, however, the main road out of the city was gridlocked as people fled the fighting, and gas stations in the area were short on fuel.

The rebels also made it into Tel Rifaat, a town near Aleppo held by U.S.-backed Syrian Kurdish forces, according to pro-government pan-Arab Al Mayadeen TV.

Tel Rifaat is far from the large swaths of northeastern Syria that are controlled by the Kurdish troops, known as the Syrian Democratic Forces. A statement from the insurgency called on the SDF to leave the areas near Aleppo that they hold and retreat to their bastions in the northeast.

Despite being at odds politically with the Syrian government, the Kurdish-led administration has opposed the insurgency and accused Turkey, the main back of Syrian opposition groups, of trying to displace the Kurdish population.

Turkish officials have said a limited offensive by the rebels was planned to stop government attacks on opposition-held areas in recent weeks, but it expanded as government forces began to retreat.

The United Nations special envoy for Syria, Geir Pedersen, said the push by the rebels poses a risk to regional security and called on resuming diplomatic efforts to end the conflict.

U.S. national security adviser Jake Sullivan told CNN’s “State of the Union” that the U.S. is watching the situation carefully. There are about 900 American troops in Syria’s northeast — far from where the insurgency is happening — who are meant to guard against a resurgence by the extremist Islamic State group.

The group leading the rebel advance is designated a terrorist organization by the U.S., and Sullivan said Washington has “real concerns about the designs and objectives of that organization.”

“At the same time, of course, we don’t cry over the fact that the Assad government, backed by Russia, Iran and Hezbollah, are facing certain kinds of pressure,” he added.

Syrian troops fortify northern Hama as jets pound Idlib

According to Syrian state news agency SANA and the Syrian Observatory for Human Rights, the army overnight pushed back insurgents in the northern countryside of Hama province.

Syrian state media said government resupply included heavy equipment and rocket launchers while Syrian and Russian airstrikes targeted weapon depots and insurgent strongholds. Pro-government radio station Sham FM said the Syrian army shot down drones belonging to Hayat Tahrir al-Sham in northern Hama.

Syrian state television claimed government forces had killed nearly 1,000 insurgents over the past three days, without providing evidence or details.

Government airstrikes in Idlib killed at least three civilians, including two children, and wounded 11 others, said the Syrian Civil Defense, known as the White Helmets, which operates in opposition-held areas.

Residents could be seen fleeing through ash-covered streets. Men lowered a child from the high floor of a building whose facade was blown off.

Airstrikes also struck in and around Aleppo, including near a hospital in the city center, killing 12 people, including at least eight civilians, according to the White Helmets and the Syrian Observatory for Human Rights.

In a call with foreign officials, Assad vowed to defeat the insurgency. “Terrorism only understands the language of force, which is the language we will break and eliminate it with, regardless of its supporters and sponsors,” he said.

The 2016 battle for Aleppo was a turning point in the war between Syrian government forces and rebel fighters after 2011 protests against Assad’s rule turned into an all-out war. After appearing to be losing control of the country to the rebels, the Aleppo battle secured Assad’s hold on strategic areas of Syria, with opposition factions and their foreign backers controlling areas on the periphery.