The US plan to rally proxy ground forces to complement its air strikes against Isis militants in Syria is in tatters after jihadis ousted Washington’s main ally from its stronghold in the north over the weekend.

The attack on the Syrian Revolutionary Front (SRF) by the al-Qaida-aligned Jabhat al-Nusra came after weeks of clashes between the two groups around the city of Idlib, which has remained one of the last bastions of regime control in northern Syria throughout the civil war.

Militants overran the command centre of the SRF’s leader, Jamal Maarouf, in Deir Sonbol in a humiliating rout that came as US and Arab air forces continued to attack Isis in the Kurdish town of Kobani, 300 miles east, in an effort to prevent the town from falling.

In Iraq, Isis has reportedly killed over 230 members of a tribe in western Anbar province in the last ten days, including dozens of women and children. The killings were some of the worst bloodshed in the country since the militants swept through northern Iraq in June.

The defeat of Maarouf is a serious blow to the US strategy of building a proxy coalition against Isis. It comes amid a groundswell of anger at the US strikes across the opposition-held north, which have done nothing to slow the intensity of attacks from Bashar al-Assad’s air force, especially in Aleppo.

“We thought the Americans were going to help us,” said an SRF spokesman. “But not only have they abandoned us, they have been helping the tyrant Bashar instead. We will move past this betrayal and get back to Jebel al-Zawiya [the group’s heartland], but it is going to take some time.”

As strikes led by the US and joined by Saudi Arabia, Bahrain and the UAE have continued to target Isis, operations by the Syrian air force have intensified across the country, with a sharp rise reported from the ground of barrel bombs – large, improvised explosives pushed from the back of helicopters that cause indiscriminate destruction.

In the past fortnight alone, at least 401 barrel bombs have been dropped on rebel areas across eight Syrian provinces, claims the Syrian Observatory for Human Rights, which monitors attacks on regime and opposition-held parts of the country. The attacks have reportedly killed more than 200 civilians and an unknown number of fighters.

At least four barrel bombs hit a refugee camp near Idlib last Wednesday in attacks that the US state department described as barbaric.

A survivor from one of the camps, Haithem Ahmed, who fled with his family to Turkey, said the regime had been emboldened by the US attacks on a common enemy and was acting with increasing impunity.

“It is obvious that the US is supporting Assad,” he said. “Don’t bother trying to argue with me or anyone else about it. They are aiding the war against us. Their leaders are weak and they are liars.”

The US defence secretary, Chuck Hagel, is reported to have warned national security adviser Susan Rice of a blowback among opposition communities in northern Syria because US strategy against Assad has not been clearly defined.

Turkey has been pushing vehemently for the establishment of a no-fly zone over parts of northern Syria, which would protect the civilian population. However, such a plan has yet to gain momentum and, if taken to the UN security council, is likely to be vetoed by Syria’s allies Russia and China.

Maarouf, a former construction worker turned leader of more than 30,000 men, had been one of the few beneficiaries of limited US military supplies to the Syrian opposition. Saudi Arabia also started to send weapons to his fighters in January 2013.

Increasingly desperate calls from Marouf and his forces had gone unanswered since Friday, and by early Saturday large numbers of SRF forces were forced to withdraw. Jabhat al-Nusra said many of Maarouf’s men had defected to its ranks.

Until it lost primacy to Isis in April last year, Jabhat al-Nusra had been the pre-eminent jihadist group among the patchwork of ideologies and agendas that comprise the Syrian opposition. Since then, al-Nusra had acted as an auxiliary group, fighting sometimes alongside the SRF and at other times with the second opposition umbrella group, the Islamic Front, which has been backed by Qatar and Turkey.

Al-Nusra had been reeling over the past six weeks since more than 50 of its members were killed in the opening salvo of US air strikes, which it had thought were intended only for Isis.

Since then, its leaders have been partly reconciled with Isis in northern Syria, sending commanders and men to fight in joint operations. There were unconfirmed reports that Isis members had joined al-Nusra in the battle for Maarouf’s village.

Elsewhere, Isis remains on the back foot in Kobani, after committing far more forces than it expected in an effort to take the town, close to the Turkish border. Kurdish Peshmerga forces from northern Iraq entered Kobani over the weekend and two heavy field guns they brought with them on the overland journey though Turkey are proving decisive in holding back Isis forces, who still occupy about half of the town.

Kobani has become a defining struggle between Isis and the US, as much as it is between the jihadis and the Kurds who, with US help, beat back an advance on Irbil in August. If Isis was able to take Kobani it could boast a significant victory. A victory over the secular Kurds would help advance its hardline interpretation of Islam, which has seen it rule areas it controls along strict medieval precepts that are rooted in an uncompromising understanding of Islamic teachings.