Al-Qaeda is creating its most powerful stronghold ever in north-west Syria at a time when world attention is almost entirely focused on the impending defeat of Isis in the east of the country. It has established full control of Idlib province and of a vital Syrian-Turkish border crossing since July. “Idlib Province is the largest al-Qaeda safe haven since 9/11,” says Brett McGurk, the senior US envoy to the international coalition fighting Isis.

The al-Qaeda-linked movement, Hayat Tahrir al-Sham (HTS), which used to be called Jabhat al-Nusra, has long been the most powerful rebel group in western Syria. After the capture of east Aleppo by the Syrian army last December, it moved to eliminate its rivals in Idlib, including its powerful former Turkish-backed ally Ahrar al-Sham. HTS is estimated to have 30,000 experienced fighters whose numbers will grow as it integrates brigades from other defeated rebel groups and recruits young men from the camps for Internally Displaced Persons (IDPs) who have sought refuge in Idlib from President Bashar al-Assad’s forces.

Al-Qaeda is growing in strength in and around Idlib province just as Isis is suffering defeat after defeat in eastern Syria and Iraq. Its latest setback was its failure on Tuesday to stop the Syrian army linking up with its enclave at Deir Ezzor, where Isis has been besieging the government held part of the city for three years. Divided by the Euphrates, the city is the largest in eastern Syria and its complete recapture opens the way to the al-Omar oilfields that once provided half of Syria’s crude production.

The end of the siege, supposing encircling Isis forces are permanently driven back, frees up a Syrian army garrison of 5,000 to 10,000 soldiers as well as the 93,000 civilians trapped in the government-held zone who had been supplied with food by airdrops. Deir Ezzor is only the latest Isis urban centre to be lost on the Syrian portion of the Euphrates valley which was the heartland of its territories in Syria. Isis is everywhere on the retreat. Upriver from Deir Ezzor at Raqqa, the American-backed and Kurdish-run Syrian Democratic Forces are fighting their way into the city and has captured its old city quarter in the last few days.

Despite its proven fighting prowess, Isis is collapsing under the impact of ground attacks launched by different parties on multiple fronts in Iraq and Syria. What tips the balance against it in all cases is the massive firepower of the Russian and US air forces in support of ground assaults. Isis’s defeat in eastern Syria will accelerate as local tribes, previously won over or intimidated by Isis, join the winning side. The US and the Syrian Kurds may not like the return of Syrian government authority in eastern Syria, south of Raqqa, but they do not look as if they are prepared to fight hard to stop it. President Trump’s priority is to eradicate Isis and al-Qaeda, regardless of who rules Syria in future.

Bad news for Isis is good news for HTS and al-Qaeda. Its defeat preoccupies its myriad enemies and largely monopolises their military efforts. Short of combat troops, the Syrian army is only really capable of making a maximum effort on one front at a time. The Syrian Kurds have an interest in fighting Isis but not necessarily defeating it so decisively that the US would no longer need a Kurdish alliance and could return to the embrace of its old Nato ally Turkey.

HTS stands to benefit politically and militarily from the decline of Isis, the original creator and mentor of Jabhat al-Nusra, as the earliest of al-Qaeda’s incarnations in Syria was known. Under the name of al-Nusra, the Syrian branch of the movement split off in 2013 and the two sides fought a bloody inter-jihadi civil war. If Isis is destroyed or rendered a marginal force, Sunni Arab jihadis refusing to surrender to Mr Assad’s army and intelligence service will have no alternative but to join HTS. Moreover, Sunni Arabs in eastern Syria may soon be looking for any effective vehicle for resistance, if Syrian government armed forces behave with their traditional mix of brutality and corruption.

HTS will expect the many states now attacking Isis, and battering to pieces its three-year-old caliphate, to turn on them next. But they will hope to delay the confrontation for as long as possible while they strengthen their movement. Ideologically similar but politically more astute than Isis, they will seek to avoid provoking a final territorial battle which they are bound to lose. Some Syrian specialists warn against waiting too long. “The international community must seek urgently to counter-attack HTS, which grows stronger by the day, without waiting for the complete destruction of the Islamic State,” writes Fabrice Balanche in a study published by the Washington Institute for Near East Studies called Preventing a Jihadist Factory in Idlib. He says that HTS wants to dominate the whole Syrian rebellion and is close to succeeding.

In Pictures: The crisis unfolding in Syria Show all 30 1 /30 In Pictures: The crisis unfolding in Syria In Pictures: The crisis unfolding in Syria A Syrian family arrives at a checkpoint, manned by pro-government forces, at the al-Hawoz street roundabout, after leaving Aleppo's eastern neighbourhoods Getty In Pictures: The crisis unfolding in Syria A Syrian woman, fleeing violence in the restive Bustan al-Qasr neighbourhood, reacts as she stands with her children in Aleppo's Fardos neighbourhood, after regime troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian pro-regime fighters, gesture as they drive past resident fleeing violence in the restive Bustan al-Qasr neighbourhood, in Aleppo's Fardos neighbourhood Getty In Pictures: The crisis unfolding in Syria Syrian rebels withdrew from six more neighbourhoods in their one-time bastion of east Aleppo in the face of advancing government troops, the Syrian Observatory for Human Rights said Getty In Pictures: The crisis unfolding in Syria Syrian rebels withdrew from six more neighbourhoods in their one-time bastion of east Aleppo in the face of advancing government troops, the Syrian Observatory for Human Rights said Getty In Pictures: The crisis unfolding in Syria Syrian residents, fleeing violence in the restive Bustan al-Qasr neighbourhood, arrive in Aleppo's Fardos neighbourhood , after regime troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian pro-regime fighters, gesture as they drive past residents fleeing violence in the restive Bustan al-Qasr neighbourhood, in Aleppo's Fardos neighbourhood Getty In Pictures: The crisis unfolding in Syria Syrian residents, fleeing violence in the restive Bustan al-Qasr neighbourhood, arrive in Aleppo's Fardos neighbourhood, after regime troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian residents, fleeing violence in the restive Bustan al-Qasr neighbourhood, arrive in Aleppo's Fardos neighbourhood Getty In Pictures: The crisis unfolding in Syria A Syrian pro-regime fighter speaks with a child, as residents flee violence in the restive Bustan al-Qasr neighbourhood. Syrian rebels withdrew from six more neighbourhoods in their one-time bastion of east Aleppo in the face of advancing government troops AFP/Getty Images In Pictures: The crisis unfolding in Syria Smoke rises as seen from a governement-held area of Aleppo, Syria Reuters In Pictures: The crisis unfolding in Syria Syrian soldiers targeting rebels-held areas in the eastern neighborhoods in Aleppo, Syria. According to media reports, the army is now holding on 99 percent of Aleppoís eastern neighborhoods EPA In Pictures: The crisis unfolding in Syria Syrian pro-government forces patrol Aleppo's eastern al-Salihin neighbourhood after troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian soldiers rest following the battle at al-Sheik Saeed neighborhood in Aleppo, Syria EPA In Pictures: The crisis unfolding in Syria A Syrian pro-government fighter walking past closed shops in the Bab al-Nasr district of Aleppo's Old City. Once renowned for its bustling souks, grand citadel and historic gates, Aleppo's Old City has been rendered virtually unrecognisable by some of the worst violence of Syria's war Getty In Pictures: The crisis unfolding in Syria The crucial battle for Aleppo entered its 'final phase' after Syrian rebels retreated into a small pocket of their former bastion in the face of new army advances. The retreat leaves opposition fighters confined to just a handful of neighbourhoods in southeast Aleppo, the largest of them Sukkari and Mashhad Getty In Pictures: The crisis unfolding in Syria Syrian civilans arrive at a checkpoint, manned by pro-government forces, at the al-Hawoz street roundabout, after leaving Aleppo's eastern neighbourhoods. Syria's government has retaken at least 85 percent of east Aleppo, which fell to rebels in 2012, since beginning its operation Getty In Pictures: The crisis unfolding in Syria Syrian civilians flee the Sukkari neighbourhood towards safer rebel-held areas in southeastern Aleppo Getty In Pictures: The crisis unfolding in Syria Syrians celebrate in the government-held Mogambo neighbourhood of the northern Syrian city of Aleppo, after rebel fighters retreated into a small pocket of their former bastion in the face of new army advances Getty In Pictures: The crisis unfolding in Syria Syrians celebrate in the government-held Mogambo neighbourhood of the northern Syrian city of Aleppo, after rebel fighters retreated into a small pocket of their former bastion in the face of new army advances. The fall of Aleppo would be the worst rebel defeat since Syria's conflict began in 2011, and leave the government in control of the country's five major cities Getty In Pictures: The crisis unfolding in Syria A Syrian refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria Syrian refugee Aliya inside the tent where she lives with her husband and ten children in a camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria Syrian refugee women and children outside the entrance to their tents in the refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA Wire In Pictures: The crisis unfolding in Syria A Syrian refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria A Syrian refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA Wire In Pictures: The crisis unfolding in Syria A Syrian refugee woman outside the entrance to the tent where her family live, in the refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria A vehicle drives past a mosque at night in Idlib, Syria. Picture taken with a long exposure Reuters In Pictures: The crisis unfolding in Syria Damaged buildings stand in the rebel-controlled town of Binnish in Idlib province, Syria Reuters In Pictures: The crisis unfolding in Syria The night sky is seen through damaged windows in the rebel-controlled town of Binnish in Idlib province, Syria Reuters In Pictures: The crisis unfolding in Syria Damaged buildings stand in the rebel-controlled area of Maaret al-Numan in Idlib province, Syria Reuters

The open dominance of an extreme Islamic jihadi movement like HTS creates a problem for foreign powers, notably the US, UK, France, Turkey, Saudi Arabia and Qatar, previous funders and suppliers of the Syrian rebels. HTS, whose attempt to distinguish itself from al-Qaeda has convinced few, is listed in many countries as a terrorist organisation, unlike its former ally, the Ahrar al-Sham. It will be difficult for foreign powers to do business with it, though the armed opposition to Mr Assad has long been dominated by extreme Islamist jihadi groups. The difference is that today there are no longer any nominally independent groups through which anti-Assad states and private donors can channel arms, money and aid while still pretending that they were not supporting terrorism.