As the Islamic State group reels from losses in Iraq and Syria, a group of US-trained Syrian rebels have attacked the jihadists at a key crossing between the two countries.

The New Syrian Army, trained and supported by the United States, eventually withdrew from the Albu Kamal border post after heavy fighting on Wednesday.

Analysts say the group is marginal and unlikely to have a significant impact against IS.

What do we know about the group, and how effective can it be?

– Ousting IS from Deir Ezzor –

The New Syrian Army was formed in November of fighters from other rebel groups, mostly from Syria's eastern Deir Ezzor province.

It is based around Al-Tanaf, another Iraqi-Syrian border crossing it seized from IS after sweeping in from Jordan in March.

The group says it wants to push IS out of the entire desert province of Deir Ezzor. It opposes the regime of Bashar al-Assad, but so far has only attacked IS positions.

The group has concentrated on seizing Albu Kamal, about halfway between Raqa, IS's de facto Syrian capital, and Baghdad. The town has been held by IS since mid-2014.

But taking on IS is a big job for such a small force.

– International support –

The New Syrian Army has no more than 150 fighters, according to Charles Lister, a Syria analyst at the Middle East Institute.

The Pentagon says about 100 of them have been through its train-and-equip programme, which aims to turn them into special forces.

"On a fighter-by-fighter basis, the US has invested in the New Syrian Army more than any other counter-ISIS force," Lister told AFP, using another acronym for IS.

The group's offensives are supported by international air strikes commanded by Washington, which considers it a "partner opposition force".

Syrian rebel commanders told The Times this month that British special forces were working with the group at its base in Al-Tanaf — a claim its commanders denied.

The group does coordinate its operations with Iraqi counter-terrorism forces, which are trying to oust IS from Al-Qaim, a nearby town on the Iraqi side of the border.

Its efforts may be hindered by the Assad regime's allies. In mid-June, Washington condemned an alleged Russian air strike against the group's headquarters in Al-Tanaf — a claim Moscow denies.

– How effective can it be? –

The goal of Wednesday's operation was to cut a crucial link between IS-held territories in Syria and Iraq along the Euphrates Valley.

The Syrian Observatory for Human Rights, a monitoring group, said the rebels had seized the small nearby Al-Hamdan airbase, but IS jihadists recaptured it by Wednesday afternoon and forced the US-backed fighters to retreat.

"The idea of dropping a small force in the middle of IS's heartlands was utterly foolish, bound to fail. The NSA is definitely too weak (a few hundred men) to engage in such dangerous operations," said Thomas Pierret, a Syria expert at the University of Edinburgh.

Others, however, say the group — the only US-backed force in southeastern Syria — can play an important role.

"Paired up with Iraqi security forces on the other side of the border, the New Syrian Army has great potential, but it takes time to grow a force to the capacity needed for such a mission," Lister said.