Egypt’s most active jihadi group, Ansar Beit al-Maqdis, has disclosed its allegiance to Islamic State, becoming one of the largest militant bodies to pledge loyalty to Isis outside its strongholds in Iraq and Syria.

In a nine-minute audio recording released early on Monday, a spokesman for ABM said: “In accordance with the teachings of the Prophet, we announce our allegiance to the Caliphate, and call on Muslims everywhere to do the same.” The message attempted to justify its pledge by alluding to violations by the Egyptian state, which has crackec down on most forms of opposition over the last year, in particular in the region where ABM is based.

The group has fought an insurgency against the Egyptian military and police in the remote towns of the northern Sinai peninsula since at least 2011, increasing its attacks after the army removed the moderate Islamist president Mohamed Morsi in July 2013.

The group are believed to be behind last month’s killing of at least 30 Egyptian soldiers near the border with Gaza, in what was one of the worst peacetime attacks on Egypt’s army in its history. Before they were arrested this spring, the group’s cells in mainland Egypt also bombed police headquarters in Cairo and Mansoura, and tried to assassinate the country’s interior minister. They are not active in the tourist hotspots of southern Sinai.

An ABM representative first swore allegiance to Isis in the late summer, two well-placed sources said. In July, a senior ABM member travelled from the group’s base in northern Sinai to Raqqa, Isis’s stronghold in north-east Syria. He spent around three months there, and eventually swore a baya, a pledge of allegiance, on behalf of ABM.

“They also agreed that Isis will train ABM’s members [in Syria] because proper training is not possible in Egypt due to the [presence of] the military,” said one source with knowledge of the representative’s visit.

The group kept the relationship largely secret until this week, as members with strong ties to Sinai’s tribes feared that disclosing the news would alienate large parts of the local population. But that decision appeared to cause divisions within ABM, with one faction publicly declaring the group’s allegiance to Isis last Tuesday, only for another to deny releasing such a statement hours later.

With the release of Monday’s audio recording, this discrepancy appears to have been resolved. But the practical effect of the pledge remains to be seen.

The announcement could give Isis sympathisers in mainland Egypt the impetus they need to join ABM, but it may alienate potential allies in Sinai, argued Zack Gold, an analyst who researches Sinai militancy.

“If a pledge to [Isis] means that ABM will … focus more on sectarianism and ‘governing’ the population of Sinai, this is likely to harm local recruitment and support in the peninsula,” said Gold, a visiting fellow at the Institute for National Security Studies. But it also “may attract Egyptians that have, on the internet, pledged support to [Isis] but had no practical outlet for that support”.

In its message, ABM specifically appealed to more moderate Islamists, such as members of the Muslim Brotherhood, some of whom may have become disillusioned with the democratic process after Morsi’s removal last year. “You will not get any use from the infidel democracy – it is better to die with honour than live in humiliation,” the recording said.

But while there are other smaller jihadi groups operating in Cairo, any Isis sympathisers would have to go to the Sinai peninsula itself to join the group, an Egyptian jihadi based in Syria suggested. “There are no Isis cells in mainland Egypt,” he said.

Additional reporting by Manu Abdo