Following significant gains in Iraq and Syria, the Islamic State group is now heading toward Aleppo, located 223 miles north of the Syrian capital of Damascus, helped along by President Bashar Assad's troops, a report said Tuesday, citing Syrian opposition leaders.

ISIS fighters clashed with rival Syrian rebels in the countryside northeast of Aleppo on Tuesday, raising fears that the extremist group is positioning itself for a major offensive to capture Syria’s largest city, the New York Times reported. Syria's opposition leaders have accused the country’s government of collaborating with ISIS militants to strike rebel positions, while anti-Assad forces called for air support from the United States to counter simultaneous attacks by government troops and ISIS.

“We appeal to them [neighboring countries] to relieve the free people of Syria through establishing safe areas so Assad’s warplanes do not serve as an air force for ISIS,” Khaled Khoja, president of the National Coalition for Syrian Revolutionary and Opposition Forces, a major Syrian opposition group in exile, said in a statement. “This is already taking place in Aleppo, as the regime’s warplanes are bombing rebels in the areas ISIS are preparing to attack. The regime’s air force is openly serving ISIS; it bombs rebels while ISIS advances,” he said.

The long-closed U.S. Embassy in Syria supported the claims on Twitter, saying that Assad’s government is essentially assisting ISIS to further strengthen its position in Syria. “Reports indicate that the regime is making air-strikes in support of #ISIL's advance on #Aleppo, aiding extremists against Syrian population,” the embassy tweeted.

Rebel groups said that the recent fighting around Aleppo served as the strongest indication, by far, of an active collaboration between Assad and ISIS forces.

“It was never this blatant,” the Times quoted Abu Abdo Salabman, a spokesman for Sham Revolutionary Brigades, a rebel group, as saying. “The whole thing started with a combination of aerial and then long-range artillery fire from the regime on the rebels. Then ISIL started their advancements. There is clear advanced coordination this time, and not just a side trying to take advantage of the other.”

However, neither the U.S. nor Syrian opposition groups have provided evidence to support claims of alleged coordination between Assad’s forces and ISIS. Meanwhile, Marie Harf, a U.S. State Department spokeswoman, said Tuesday that officials were investigating the claims but had no confirmation yet, the Times reported.

“Beyond that, we have long seen that the regime avoids ISIL lines, in complete contradiction to the regime's claims publicly to be fighting ISIL,” Al Jazeera quoted Harf as saying. “As we have long said, Bashar Assad has lost legitimacy long ago and will never be an effective counterterrorism partner, despite what he might say publicly.”