US air strikes in Syria are encouraging anti-regime fighters to forge alliances with or even defect to Islamic State (Isis), according to a series of interviews conducted by the Guardian.

Fighters from the Free Syrian Army (FSA) and Islamic military groups are joining forces with Isis, which has gained control of swaths of Syria and Iraq and has beheaded six western hostages in the past few months.

Some brigades have transferred their allegiance, while others are forming tactical alliances or truces. Support among civilians also appears to be growing in some areas as a result of resentment over US-led military action.

“Isis now is like a magnet that attracts large numbers of Muslims,” said Abu Talha, who defected from the FSA a few months ago and is now in negotiations with other fighters from groups such as the al-Nusra Front to follow suit.

Assam Murad, a fighter from a 600-strong dissident FSA brigade near Homs said: “There’s no way we would fight Isis after the US military campaign against them.”

A third man, Abu Zeid, the commander of an FSA brigade near Idlib and a defector from President Bashar al-Assad’s army, said: “All the locals here wonder why the US coalition never came to rescue them from Assad’s machine guns, but run to fight Isis when it took a few pieces of land. We were in a robust fight against Isis for confiscating our liberated areas, but now, if we are not in an alliance, we are in a truce with them.”

These and other Syrian fighters told the Guardian in interviews by phone and Skype that the US campaign is turning the attitudes of Syrian opposition groups and fighters in favour of Isis. Omar Waleed, an FSA fighter in Hama, north of Damascus, said: “I’m really scared that eventually most of the people will join Isis out of their disappointment with the US administration. Just have a look on social media websites, and you can see lots of people and leaders are turning to the side of Isis.

“We did not get any weapons from the US to fight the regime for the last three years. Only now US weapons arrived for fighting Isis.”

Abu Talha said he had joined the FSA after being released from prison in an amnesty Assad granted shortly after the Syrian uprising began in March 2011, and became commander of the Ansar al-Haq brigade in Ghouta, an eastern suburb of Damascus. He became disillusioned with the FSA, however, believing it was a tool of foreign intelligence services and poor in combat. After four senior fighters in his brigade were fatally wounded a few months ago, he defected to Isis.

“Since that day, I vowed not to fight under a flag bearing the mark of the FSA even for a second. I looked around for truthful jihadis, to fight by their side. I could not find any better than the jihadis of Isis. I told my fighters: ‘I’m going to join Isis, you are free to follow me or choose your own way’,” he said.

More than 200 of his fellow fighters also declared their allegiance to Isis, a move met with opprobrium by other FSA brigades and civilians. Then the US and its allies began a campaign of air strikes.

“All those who were cursing and attacking us for joining Isis came to pledge their loyalty to Isis. A couple were FSA commanders, others were members of Islamic brigades. Even ordinary people now demand to be governed by Isis,” Abu Talha said.

Only a small number openly declared their new allegiance, he added. “Large brigades in Idlib, Aleppo, Derra, Qalamoun and south Damascus have pledged loyalty to Isis in secret. Many senior leaders of brigades in Syria are in talks with us now to get together and fight as a united force against the US aggression,” he said. His claims cannot be independently verified.

Murad, a fighter with the FSA’s 600-strong al-Ribat brigade near Homs, said an offer three months ago by the US-backed Hazem movement to supply his unit with advanced weaponry if it joined the fight against Isis was turned down.

“We rejected this attractive offer, even though we are in great need not only of weapons but food. There is no way that we would fight Isis after the US military campaign against them,” he said.

He and his fellow fighters were awaiting the arrival of Isis militants in Homs, he added. “The moment Isis fighters touch the soil of the Homs countryside, we will be the first to fight with them at the front. This [US-led] military coalition is not against Isis, it is against entire Islam.”

Fighters from Islamic militias are also joining forces with Isis. In Idlib, in north-west Syria, the Jaish al-Mujahideen army, al-Sham brigade, Ahrar al-Sham brigade and al-Nusra Front were all in conflict against Isis earlier this year. Now they are calling for an alliance. More than 1,000 al-Nusra Front fighters in the area joined forces with Isis in a single week in August, according to Ali Sa’eed, a spokesman for the FSA revolutionary command in Idlib.

Abu Talha said he was in talks with al-Nusra Front leaders, “asking them to proclaim their allegiance to Isis and be one hand to defeat Bashar [al-Assad] and all the tyrants in the world”.

“There are senior leaders of al-Nusra Front who are waiting for the zero hour to unite with us. They are more conscious now of the great risks that lie behind the new US crusade against Muslims and jihadis,” he said.

According to those interviewed, civilians as well as fighters are turning towards Isis. The group is gaining support because it implements social measures and increases security, according to Abu Talha.

“We opened 57 free public restaurants in Raqqa city, which provide three meals a day for any resident to foil any claim by a looter that he had to steal in order to feed his children. We provide free fuel to residents as well.” The implementation of sharia law had led to a huge fall in the crime rate in Raqqa and other cities controlled by Isis, he said.

In Ghouta, near Damascus, the al-Nusra Front is the dominant force, but it has lost ground to a few hundred Isis fighters, according to locals. “I can assure you the day Isis declares they are coming to Ghouta, all the people and brigades will be with them out of our dismay and disappointment,” said Fadhil Ali, a restaurant worker. “We can’t wait for the day we have Isis in Ghouta.”

Isis does not have enough weapons for the number of foreign and local jihadis wanting to join its ranks, Abu Talha said. “Jihadis in Algeria, Morocco and Yemen are declaring their allegiance to Isis. Soon we will be in Gaza and then in Iran. People are starting to be aware that Isis is defending the Sunnis.”

The growth in support for Isis was inevitable, he said. “People are suffocated and cannot stand any more. Even when you push a small cat to a corner, it will scratch you. They are slaughtering and killing us. Why should we be silent about it?”