The Hazm movement had been seen as the ‘poster boy’ of the moderate opposition Washington hopes can be trained to oust President Bashar al-Assad

US policy towards Syria has suffered a new blow with the dissolution of the Hazm movement, its favoured and best-known rebel group – raising tough questions about Washington’s strategy and limiting its future options.

Hazm (“Determination”) announced its demise at the weekend after fierce battles with Jabhat al Nusra (JAN), the al-Qaida-linked group and jihadi rival of the Islamic State (Isis) that is fighting Bashar al-Assad, the Syrian president. It said it wanted to avoid further bloodshed after clashes with JAN killed dozens of men on both sides.

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Hazm is one of several dozen Syrian rebel groups that have received US anti-tank Tow missiles and training in the past and has been described as the “poster boy” for the moderate opposition at a time when attention is focused sharply on Isis. But other elements of the Free Syrian Army (FSA), such as Fursan al-Haq, still receive discreet American support.

“People are misrepresenting this as Hazm being the last of the moderates,” said independent analyst Malik al-Abdeh. “It was like a boy band created by Simon Cowell – a dream team of moderates put together by the US and others working with it. But people don’t really care that much about them.”

Video footage showed JAN fighters posing with Tow missiles captured when it overran Hazm HQ in Idlib.

Tension between Hazm and JAN – whose members have been bitter enemies for a long time – has been on the rise for months after Hazm sided with another FSA group, the Syria Revolutionary Front, when JAN was fighting that militia and expelled it from Idlib province. But Syria watchers say that Hazm provoked JAN by kidnapping and killing some of its top leaders, including its emir, Abu Eissa al-Tabqa.

Hazm has said it will now join forces with the Shamiah Front, formed to defend besieged rebel-held areas of Aleppo, after Assad launched an offensive to encircle them. The Shamiah front includes hardline Salafist factions as well as more moderate brigades like the Muslim Brotherhood-linked Mujahideen Army and another US-backed outfit.

From Washington’s point of view, the timing of the Hazm collapse was bad, coinciding with the long-awaited start of a US-sponsored training programme for moderate rebels in Turkey. The plan is to train and equip a new force as part of a broader plan to develop ground forces needed to defeat Isis, which is currently being targeted only from the air in Syria. The US worked with Saudi Arabia and Jordan to develop the plan. Qatar has also joined the programme.

The US has said that 1,200 Syrian rebels from moderate factions have already been screened and vetted but the bulk of the forces are expected to be drawn from Syrian refugees in Turkey and Jordan. The most effective rebel forces are in southern Syria, where FSA units have been far more effective in marshalling and organising their forces with the help of a command centre in Jordan.

President Barack Obama’s critics look likely to seize on the fate of Hazm to urge an abandonment of support for rebels in the north, now dominated by JAN and by Ahrar al-Sham, which is also beyond the pale as far as the US is concerned. “It makes US options for the north more limited than ever,” said Abdeh.

“People in DC will use this to argue that the train-and-equip programme won’t work and that US weapons will fall into the hands of JAN and that they should just focus on the south where the FSA is stronger – or give up supporting the rebels altogether and back the UN work on local ceasefires to de-escalate the conflict. This will strengthen those arguments.”