The US has warned that the takeover of a rebel-held province in Syria by militants with links to al-Qaeda will have “grave consequences” for the future of the region.

A letter in Arabic from Michael Ratney - the US State Department’s top Syria policy official - posted online late on Wednesday said that the recent offensive by Hayat Tahrir al-Sham (HTS), spearheaded by former al-Qaeda offshoot Nusra Front ( now called Jabhat Fatah al-Sham), which cemented its grip on Idlib province, had put “the future of northern Syria in big danger”.

Idlib is the only region of the country totally under rebel control after more than six years of fighting which has seen the tide of the war turn in Syrian President Bashar al-Assad’s favour.

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It has become an overcrowded home for more than two million people who have been moved there following the end of sieges and government offensives in the rest of the country. The fall of Aleppo in December 2016 in particular saw a huge influx of rebels and their families.

Syrian regime and Russian air strikes have heavily targeted Idlib, killing hundreds of civilians, since Islamist rebels swept through it in 2015.

It is currently one of four de-escalation zones in the country following a Russian-Turkish ceasefire brokered in May.

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In a three day blitz at the end of last month, however, Abu Mohammad al-Jolani, HTS’ de facto leader, managed to seize control of most of Idlib from the more mainstream Ahrar al-Sham in some of the heaviest inter-rebel fighting seen in the complex conflict to date.

Nusra and its leaders would remain a target of Washington even if they adopted new names in an attempt to deny the US and other powers a pretext to attack them, the diplomat said.

The HTS takeover last month could also make it difficult for US diplomats to convince their Russian counterparts not to renew a bombing campaign, he added.

Calling the successful HTS offensive one of north Syria’s “biggest tragedies”, Mr Ratney wrote that “In the event of the hegemony of Nusra Front on Idlib, it would be difficult for the United States to convince the international parties not to take the necessary military measures.”

Mr Ratney urged other rebel groups in the region not to join HTS' proposed civil and military joint administrations.

“Everyone should know that Jolani and his gang are the ones who bear responsibility for the grave consequences that will befall Idlib,” he added.