At least two Russian fighters were killed in US air strikes in north-eastern Syria last week, their associates have told the BBC.

The mercenaries are said to have been hired by private military firms backing Syrian pro-government troops.

Russia has not confirmed the deaths that were first reported by US media, saying such reports should not be treated as "primary sources".

The US says it killed more than 100 fighters in the strikes last week.

But Defence Secretary Jim Mattis insisted that despite the media reports he did not have information that "Russian contractors" were among the casualties,

"I can't give you anything on that, we have not received that word at Central Command or the Pentagon," he said.