The west’s policy on Syria has been thrown into disarray due to sweeping advances by al-Qaida-linked militants in the north-west of the country, gaining the military upper hand in the largest area of opposition-held territory.

The assertion of control by Hayat Tahrir al Sham (HTS), the former al-Qaida affiliate previously known as the al-Nusra front and Jabhat Fateh al-Sham, over the province of Idlib amid the scaling back of American support for rebel groups has led to fears that Assad’s allies, including Moscow, would use the move as a pretext for a broad and devastating military campaign.

“The future of the north is in great danger,” said Michael Ratney, the US state department’s Syria envoy, in a statement posted online. “If [Hayat Tahrir al Sham’s] control of Idlib is realised it will be difficult for the US to convince other international parties to refrain from necessary military measures.”

A western diplomat said there was no evidence yet that either Russia or the Syrian government were gearing up for a broad military offensive on Idlib. But the stark acknowledgment by the US that it could be unable or possibly unwilling to prevent such a campaign was the clearest sign yet of the alarm at the militants’ gains and that it sees no benefit to continuing to overtly back the rebellion against the regime of Bashar al-Assad, giving Moscow greater leverage over the outcome of the conflict.

Idlib was seized by a group of largely Islamist opposition fighters supported by US-backed rebels in a major campaign in the spring of 2015. The two most powerful groups in the province were the Islamist Ahrar al-Sham and HTS.

Last week, HTS seized a key border crossing with Turkey and routed Ahrar al-Sham from several key areas in the province in some of the worst inter-rebel fighting since the start of the six-year long uprising against Assad.

The attempt by the al-Qaida-linked militants to establish overt control over Idlib was met with protests by civilians, who urged them to withdraw from major cities and allow civilian control over local government. Many fear the militants’ rise will offer a pretext for Assad and his allies to bomb the province in a manner similar to their violent reclamation of the city of Aleppo late last year.

“They want to finish us and Jolani and his gang were the tools,” said an Ahrar al-Sham source, referring to Abu Mohammad al-Jolani, the leader of the al-Nusra front.

There are around 2 million people in Idlib, many of them internal refugees who fled the fighting elsewhere in the country after local ceasefire agreements. It is believed that 1.3 million are in need of humanitarian aid.

The US recently cut aid to rebels through a covert CIA training programme and the Donald Trump administration has shown an increasing willingness to cede Syria’s fate to Assad’s allies in Moscow. But as if to underscore the contradictions in American policy, its secretary of state, Rex Tillerson, said on Tuesday that Washington was still committed both to Assad’s departure and cooperation with Russia.

“Clearly, Russia has aligned itself early on in the conflict with the Syrian regime and Bashar al-Assad, which we find to be unacceptable,” he said in a press briefing. “And it continues to be our view that the Assad regime has no role in the future governing of Syria.”

A western diplomat said the HTS move played into the Assad regime’s narrative that it was fighting terrorists, but that civil resistance to the militants was key because it was “unclear what HTS intends, and they have made no attempt yet at governance”.

The US has worked with Russia to mediate local ceasefires in parts of Syria, including a recent agreement to set up a “de-escalation” zone in the south where rebels near the Jordanian border are battling the Assad regime. The US continues to back efforts to fight the Islamic State terror group, with a campaign to reclaim the militants’ de facto capital of Raqqa under way and led by Kurdish paramilitaries.