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The US confirmed there would be be no cooperation with the Russian military in Syria - at least for now.

US Defence Secretary Jim Mattis made the comments on Tuesday, confirming the only current coordination between the US-led coalition with Russia in Syria is through a special hotline to make sure there are no mishaps involving the two sides' ground forces or planes. 

"We will not be doing anything additional until the Secretary of State (Mike Pompeo) and the president have further figured out at what point we are going to start working, alongside our allies, with Russia in the future," Mattis said at a press conference in California. 

"That has not happened yet. And it would be premature for me to go into any more detail at this point, because we're not doing any more than this."

The comments came after US President Donald Trump met with Russian President Vladimir Putin in Helsinki, Moscow last week and put forward plans to Washington to cooperate on the safe return of refugees to Syria.

On Friday, Pompeo confirmed this had been part of the presidents' talks, saying “there was a discussion between President Trump and President Putin about the resolution in Syria and how we might get the refugees back.”

"It's important that at the right time, through voluntary mechanism, the refugees are able to return to their home country,” Pompeo told reporters at UN headquarters in New York.

"There is lots of work to do to figure out how to implement that, but the United States certainly wants to be part of helping to achieve that resolution in Syria.”

But US defence officials are aghast at the prospect of coordinating with Russia in Syria, where the two nations are conducting two separate military campaigns on opposing sides.

Not only would such a move require special permission from Congress, but the Pentagon blames Russia for many civilian deaths and contributing to the conditions that caused refugee flows in the first place.

On Thursday, General Joe Votel, the top commander overseeing US military involvement in Syria, said he'd received no new instructions to work with Russians since Trump's summit with Putin.

Congress passed a law banning military-to-military cooperation after Russia annexed Crimea in 2014.

Washington and Moscow back opposing sides in the Syrian war, with Russia's intervention in 2015 in support of the brutal Assad regime being widely seen as a turning point in the multi-front conflict.

The Syrian conflict began when the Baath regime, in power since 1963 and led by Assad, responded with military force to peaceful protests demanding democratic reforms, triggering an armed rebellion fuelled by mass defections from the Syrian army.

According to independent assessments, brutal tactics pursued mainly by the Russian-backed regime, which have included the use of chemical weapons, sieges, mass executions and torture against civilians, amount to war crimes.