Defence Minister James Mattis addresses the press during a NATO defence ministers' meetings at the NATO headquarters in Brussels on February 15, 2017.

The United States is concerned about Russia's presence in Afghanistan and its interactions with Taliban fighters, U.S. Defense Secretary Jim Mattis said on Friday.

"We have seen Russian activity vis-à-vis the Taliban," Mattis said during a news conference in London.

"I'm not going to say at this point if that has manifested into weapons and that sort of thing, but certainly what they're up to there in light of their other activities gives us concern."

Mattis added that he had not yet decided whether to recommend an increase in U.S. troop numbers in Afghanistan.

