US reaches out to Iran on Syria

US and Western diplomats are scrambling to cobble together a diplomatic strategy to end the war in Syria after the latest humiliating blow to their military plan.

US Secretary of State John Kerry and his European counterparts reached out to traditional foe Iran on the sidelines of the UN General Assembly in New York on Saturday.

Iran and Russia back Syrian strongman Bashar al-Assad, whom Washington sees as the instigator of a civil war that has left half his country in the hands of the Islamic State group.

Unwilling to countenance a peace process that would leave Assad in power after he alienated or killed so many of his people, the US has backed small "moderate" rebel groups.

But that strategy appeared in tatters on Saturday after the Pentagon admitted the latest US-trained fighters to cross into Syria had given a quarter of their gear to al-Qaeda.

A previous 54-strong group that crossed into northern Syria earlier this year was attacked by al-Qaeda's local franchise, the Al-Nusra Front, and fell apart, leaving only four or five guerrillas active.

In contrast to the lacklustre US effort to shore up its allies on the ground, Russia and Iran have proven good friends to Assad, who clings to power in Damascus.

Iran has dispatched Shi'ite militia fighters trained and helped by its Revolutionary Guard, and Russia has deployed a powerful military presence to a base on Assad's territory.

With the initiative falling away, Kerry and his allies came to New York hoping to persuade Tehran and Moscow to push for a broad political solution to the conflict.

Kerry and European diplomatic chief Federica Mogherini met separately with their Iranian counterpart Mohammad Javad Zarif to make the case.

"I view this week as a major opportunity for any number of countries to play an important role in trying to resolve some of the very difficult issues of the Middle East," Kerry said at a joint appearance with Zarif.

According to an EU statement, Zarif told Mogherini that Iran will assist United Nations-led efforts to broker a political compromise that would end the civil war in Syria.

Kerry is also due to meet Russian Foreign Minister Sergei Lavrov on Sunday, and President Barack Obama will see President Vladimir Putin during a packed UNGA program.