Strikes said to be from planes flying out of Egyptian airbases signal step towards direct action in conflict by other Arab states

US officials claim the United Arab Emirates and Egypt were behind several air strikes on Islamist militias in Libya last week, in what would be an escalation of a regional power-play between Islamists and opposing governments across the Middle East.

UAE pilots flying out of Egyptian airbases allegedly twice targeted Islamist fighters vying for control of the Libyan capital, Tripoli, last week, US officials claimed to the New York Times and later to the AFP news agency. Speaking to the Guardian, a US official confirmed the reports were plausible.

The air strikes failed to stop Islamist militias from capturing Tripoli later in the week and announcing a new breakaway regime, forcing Libya's elected government to flee to the eastern city of Tobruk.

But successful or not, the strikes' alleged origins are a watershed moment. They suggest that a block of Middle Eastern countries led by the UAE are seeking to step up their opposition to the Islamist movements that have sought to undermine the region's old order since the start of the Arab spring in 2011.

Last summer, Egypt's military ousted the Muslim Brotherhood – an Islamist group – and has since been cracking down internally on its activities, a tactic pursued for years in the UAE.

If the US allegations are true, both countries now want to expand the campaign beyond their borders, seeking to curb the rise of brotherhood-affiliated militias threatening to take over Libya. The move could turn Libya into a proxy war between the country's elected government, backed by UAE and Egypt, and Islamists backed by Qatar, another Gulf state.

Since 2011, UAE and Qatar have played pivotal and opposing roles in Libya's political scene. The UAE is home to Libya's foremost nationalist politician, Mahmoud Jibril, while its rival Qatar hosts Ali Salabi, arguably Libya's most influential Islamist and a backer of the Muslim Brotherhood. From their respective exiles, Jibril and Salabi have directed an Islamist-nationalist rivalry that has been at the heart of Libya's politics since the death of Gaddafi in October 2011.

Salabi's Qatari allies trained the Islamist February 17 militia which is now battling nationalist forces in the eastern Libyan city of Benghazi. Until now, the UAE had provided less overt support – but its alleged air strikes would suggest it is now increasing its involvement.

On Tuesday, the US would not confirm the provenance of the air strikes on the record. Jen Psaki, a state department spokesman, criticised any external military intervention. "We believe outside interference exacerbates current divisions and undermines Libya's democratic transition," said Psaki.

A senior Egyptian military source denied Egypt's involvement, as did Egypt's foreign ministry. "We already issued two statements on this," said Badr Abdellatty, Egypt's foreign ministry spokesman, referring to comments made by the Egyptian government over the weekend. "That's all we're going to say."

In one of the statements, Egypt denied that "Egyptian military aircraft [had been] carrying out air strikes in areas controlled by armed militias in the Libyan capital, Tripoli". But the wording stopped short of denying any Egyptian involvement whatsoever.

Egypt's foreign minister, Sameh Shoukry, later denied Egyptian involvement, but agreed to provide Libyan troops with military training.

When asked about the air strikes, a Libyan cabinet minister expressed surprise at the reports of their provenance, and said that Libya did not want direct military intervention. But Habib al-Amin, Libya's culture minister, said the international community needed to provide more logistical and diplomatic support to his government.

"The international community until now has not been serious about helping the government, the legal authority in Libya, and the Libyan people," said Amin after discussing the Libyan civil war with Egyptian officials in Cairo.

"Libya is now in a civil war. And the international community is just watching. Tripoli is half-destroyed. Half of Benghazi is destroyed. What does the world want? To see the whole country destroyed?"

In an interview with the Guardian on Monday, Libya's foreign minister, Mohamed Abdel Aziz, claimed his government did not want foreign military intervention.

But he said Libya's government is now unable to safeguard key state institutions by itself, and called for "arms and any other equipment … that could ensure the possibility of protecting our strategic sites, our oil fields, our airports" against militias "who are now stronger than the government itself, and who do now possess arms even more sophisticated than the government itself".

And while Abdel Aziz ruled out requesting foreign air strikes against the insurgents in the short term, he hinted that they were a possibility should negotiations with the rebels fail. "Once we cannot achieve a serious or meaningful dialogue among all the factions, perhaps we can resort to other means afterwards," said Abdel Aziz, who was at a Cairo conference for regional foreign ministers about the future of Libya.

The contested strikes came in two waves. The first arrived on 17 August and hit more than a dozen targets in Tripoli held by Operation Dawn, a coalition of Islamist militias and their allies from the city of Misrata battling against nationalists for control of the city. The precision of the attacks immediately triggered speculation that foreign jets were involved, prompting denials from France, Italy and Nato.

A second wave hit in the early hours of 23 August, killing 17 Misratans and hitting ammunition dumps and the interior ministry building, captured by the Misratans the day before.

Evidence of foreign involvement in the strikes continued to pile up through the weekend when fragments said to be from a US-made Mark 82 bomb were found amid the wreckage. Libya's own government said it believed the bombing was the work of foreign jets.

Air attacks in support of nationalist forces have been pounding Islamist brigades in Libya for the past three months, but the targets are in Benghazi, 400 miles east of the capital. Libyan planes have neither the range, nor the mid-air refueling capability, to strike the capital from bases in the east, and lack night-bombing equipment.

Questions remain about who provided the intelligence that allowed the jets to hit key military targets, with some speculation it was the work of the so-called Unit 27, a commando unit trained by American special forces at a base of the same name outside Tripoli. Members of the unit were thought to have provided the Libyan component for a Delta Force raid in Tripoli last October which captured terror suspect Anas al-Libi.

Diplomats meanwhile worry that the bombing has transformed Libya's conflict into a regional war, inviting Qatar to respond in kind and complicating efforts by the UN to mediate a ceasefire.