KABUL (REUTERS) - The head of US and international forces in Afghanistan refused to deny reports that Russia is sending weapons to the Taleban.

General John Nicholson's comments came on Monday (April 24) after a senior US military official claims intelligence showed that Moscow is supporting insurgents, providing weapons including machine guns and that the supply has been ramped up significantly in the past 18 months.

It comes as Jim Mattis makes his first visit to Afghanistan as US Defence Secretary, prompting a stern message.

"We're going to have to confront Russia where what they're doing is contrary to international law."

Mattis added that this is going to be another tough year ahead in the fight against the Afghan Taleban and ISIS.

Moscow has been critical of Washington's handling of the war in Afghanistan where the Soviets fought their own bloody war in the eighties.

Russia has previously denied providing any weapons or cash to the Taleban, but has said it maintains ties with the group in a bid to push for peace talks.