America's top general in Afghanistan has said there was no doubt that Russia was providing weapons to the Taliban.

In the strongest statement yet over Russia's apparent re-engagement in the Afghanistan war, Gen John Nicholson said he would "not refute" that Moscow's involvement included giving weapons to the Taliban.

He was speaking in Kabul alongside James Mattis, the US defence secretary, who said that America needed to confront Russia over its actions in Afghanistan, where the Soviet Union fought and lost a bloody war in the 1980s.

"We'll engage with Russia diplomatically," Mr Mattis said. "We'll do so where we can, but we're going to have to confront Russia where what they're doing is contrary to international law or denying the sovereignty of other countries.

"For example any weapons being funneled here from a foreign country would be a violation of international law."