Air strikes continued to target Islamic State (Isis) positions near the Kurdish town of Kobani and hubs across north-east Syria on Sunday, as the terror group moved towards a new alliance with Syria’s largest al-Qaida group that could help offset the threat from the air.

Jabhat al-Nusra, which has been at odds with Isis for much of the past year, vowed retaliation for the US-led strikes, the first wave of which a week ago killed scores of its members. Many al-Nusra units in northern Syria appeared to have reconciled with the group, with which it had fought bitterly early this year.

A senior source confirmed that al-Nusra and Isis leaders were now holding war planning meetings. While no deal has yet been formalised, the addition of at least some al-Nusra numbers to Isis would strengthen the group’s ranks and extend its reach at a time when air strikes are crippling its funding sources and slowing its advances in both Syria and Iraq.

Al-Nusra, which has direct ties to al-Qaida’s leader, Ayman al-Zawahiri, called the attacks a “war on Islam” in an audio statement posted over the weekend. A senior al-Nusra figure told the Guardian that 73 members had defected to Isis last Friday alone and that scores more were planning to do so in coming days.

“We are in a long war,” al-Nusra’s spokesman, Abu Firas al-Suri, said on social media platforms. “This war will not end in months nor years, this war could last for decades.”

In the rebel-held north there is a growing resentment among Islamist units of the Syrian opposition that the strikes have done nothing to weaken the Syrian regime. “We have been calling for these sorts of attacks for three years and when they finally come they don’t help us,” said a leader from the Qatari-backed Islamic Front, which groups together Islamic brigades. “People have lost faith. And they’re angry.”British jets flew sorties over Isis positions in Iraq after being ordered into action against the group following a parliamentary vote on Friday.

David Cameron has suggested he might review his decision to confine Britain’s involvement to Iraq alone, but for now the strikes in support of Kurdish civilians and militants in Kobani were being carried out by Arab air forces from Saudi Arabia, Jordan, the UAE and Bahrain.

The US was reported to have carried out at least six strikes in support of Kurdish civilians near the centre of Kobani, where the YPG, the Kurdish militia, is fighting a dogged rearguard campaign against Isis, which is mostly holding its ground despite the aerial attacks.

Kobani is the third-largest Kurdish enclave in Syria, and victory for Isis there is essential to its plans to oust the Kurds from lands where they have lived for several thousand years. Control of the area would give the group a strategic foothold in north-east Syria, which would give it easy access to north-west Iraq.

US-led forces are also believed to have carried out air strikes on three makeshift oil refineries under Isis’s control.

Isis continued to make forays along the western edge of Baghdad, where its members have been active for nine months. The Iraqi capital is being heavily defended by Shia militias, who in many cases have primacy over the Iraqi army, which surrendered the north of the country.

That rout – one of the most spectacular anywhere in modern military history – gave Isis a surge of momentum and it has since seized the border with Syria, menaced Irbil, ousted minorities from the Nineveh plains and threatened the Iraqi government’s hold on the country.

Barack Obama said the intelligence community had not appreciated the scale of the threat or comprehended the weakness of the Iraqi army. In an interview on CBS’s 60 Minutes, he said: “Over the past couple of years, during the chaos of the Syrian civil war, where essentially you have huge swaths of the country that are completely ungoverned, they were able to reconstitute themselves. And so this became ground zero for jihadists around the world.”