"For example, any weapons being funneled here from a foreign country would be a violation of international law," said Mattis.
Asked if he would refute claims that Russia is providing weapons to the Taliban, Nicholson answered definitively.
"Oh no, I’m not refuting that," the general said.
Nicholson said the U.S. has continued to receive reports of Russian assistance to the Taliban.
“We are under no illusions about the challenges associated with this mission,” said Mattis, who called the Taliban a "barbaric enemy" as he referenced an attack Friday that killed more than 100 at an Afghan military base.
"If they come here to Afghanistan, they will be destroyed. In keeping secretary’s intent, they will be annihilated," said Nicholson, who noted that ISIS attempts to gain a foothold in the country have been mostly repelled.
"We’re going to keep going until they’re defeated in 2017," said Nicholson. "Now they have an aspiration, I think, to move fighters here from Syria. We haven’t seen it happen. And, in fact, by reducing their sanctuary here, by annihilating them here, it should very clear to ISIS main there is no space to come to in Afghanistan.”