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

Secretary of State Mike Pompeo confirmed Friday this had been part of the presidents' talks.

But US defense officials are aghast at the prospect of coordinating with Russia in Syria, where Russia and the US are conducting two separate military campaigns.

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

For the moment, the only 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."

General Joe Votel, the top commander overseeing US military involvement in Syria, said Thursday 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.