Former secret service agent Dan Bongino has said on Fox News that "nobody's been stronger on Putin than Trump".

Mr Bongino was talking with reference to the death of Russian mercenaries in Syria, saying the American military "had the Russians scrambling".

"I don't think Putin expected Trump to respond that way," he said.

"That was not an insignificant incident over there."

He also said Mr Trump's comments about Russian gas pipelines was another example of the US president confronting the Russians.