GAZIANTEP, Turkey — Syrian rebels appealed for U.S. airstrikes to counter a new offensive by the Islamic State in the northern province of Aleppo that could reshape the battlefield in Syria.
The surprise assault, launched over the weekend, opened a new front in the multi-pronged war being waged by the extremist group across Iraq and Syria, and it underscored the Islamic State’s capacity to catch its enemies off guard.
The push — which came on the heels of the miltants’ capture of the Syrian city of Palmyra and the western Iraqi city of Ramadi late last month — took them within reach of the strategically vital town of Azaz on the Turkish border.
The offensive reinforces the impression that the Islamic State is regaining momentum despite more than eight months of U.S. led-airstrikes.
Rebel groups rushed reinforcements to the farmland north of the contested city of Aleppo after the Islamic State seized five villages. As rebel fighters in jeeps and pickup trucks hurtled toward the front lines, civilians fled in the opposite direction, seeking refuge closer to the Turkish border.
Videos posted on social media accounts allied with the Islamic State showed the group in control of checkpoints in the small town of Sawran. One image showed four decapitated heads tossed into the back of a truck.
Azaz controls access to one of the most important border crossings between Syria and Turkey. If the town were to fall, the supply lines to Aleppo city would be cut and the entire rebel presence in the province would be jeopardized, rebel commanders said.
“Automatically, the Islamic State would gain control of Aleppo city,” said Abu Mohammed, the nom de guerre of a leader of the rebel group Thuwar al-Sham, based in the Turkish town of Gaziantep. “The situation is dire.”
If the Islamic State seized the area, it would also extend its reach along the Turkish border, amplifying its capacity to secure supplies and smuggle in foreign fighters at a time when Turkey’s government has imposed severe restrictions on travelers along its 580-mile border with Syria.
The specter of a fight for Azaz drew comparisons with the battle last fall for the border town of Kobane, 80 miles away, where Kurdish fighters aided by U.S. warplanes withstood a fierce assault by the Islamic State.
But U.S. intervention on behalf of rebels in the Aleppo area would probably be complicated by the presence of al-Qaeda-affiliated Jabhat al-Nusra alongside more moderate rebel groups.
The rebels have asked the international coalition to carry out strikes and have presented coordinates of Islamic State positions, “but so far we have heard nothing,” Abu Mohammed said.
The battle brought into focus the complexity of the war in Syria, where the Islamic State, the government of President Bashar al-Assad, the opposition and Syrian Kurds are fighting .
The opposition had been on the brink of launching an offensive to push weakened government troops out of the loyalist-
controlled portion of Aleppo city and were unprepared for the attack on their northern flank.
The Islamic State offensive coincided with a wave of barrel bombings by the government over the weekend that reportedly killed more than 100 people, threw rebel ranks into disarray and drew allegations of coordination between Assad’s regime and the extremist militants.
Col. Mohammed al-Ahmed, a spokesman for the main rebel coalition in Aleppo, Jabhat al-Shamiya, alleged a “bargain” was struck between the government and the Islamic State to sabotage rebel plans to seize Aleppo.
“The coordination . . . is clear proof that the two partners, the Islamic State and Assad, are on one side against the mujahideen revolutionaries in Syria,” he said.
In a series of tweets, the U.S. Embassy in Syria echoed the allegation. “Reports indicate that the regime is making air strikes in support of ISIL’s advance on Aleppo,” the embassy’s Twitter account said. The Islamic State is also known as ISIL or ISIS.
The Syrian government has repeatedly denied charges that it coordinates with the Islamic State.
A rebel fighter in the Azaz area who goes by the name Jarrah speculated that the Islamic State had sought to expand its presence into rebel-held areas of Syria to evade U.S. air attacks. “They are suffering from airstrikes in eastern Syria, so they want to come here,” he said, speaking by telephone.
Rebel leaders said Monday that they had halted the Islamic State advance and were fighting to wrest back control of Sawran, the easternmost village captured.
The Islamic State, however, has been aided by a fresh influx of weapons captured from the fleeing Syrian army at Palmyra, and the rebels have been unable to muster enough reinforcements to match the militants’ numbers, Jarrah said. “It’s going to be a big battle,” he predicted.
The Islamic State had controlled Azaz and was present throughout Aleppo until a revolt by rebels early in 2014 forced it to withdraw to the eastern part of the province. Since then, the front line between the groups had been mostly quiet until the weekend offensive.