"Any weapons being funneled here from a foreign country would be a violation of international law unless they were coming to the Government of Afghanistan," Mattis said, speaking during his first visit to Afghanistan as Defence Secretary. He added that it would have to be dealt with as such.
In the past, Nicholson has criticised Russia's contact with the Taliban, saying that it has given "legitimacy" to a group that has undermined the elected Government in Kabul.
In the 1980s, Russia fought its own war in Afghanistan, losing thousands of troops to insurgents supplied with advanced US weaponry, such as shoulder-fired surface-to-air missiles.
In March, when the commander of US forces in Europe, Army General Curtis Scaparrotti, told lawmakers that Russia was providing support to the Taliban, the Russian Foreign Ministry dismissed the allegations as "a lie" and said the charge was being promulgated to disguise Washington's own policy failures in Afghanistan.
Mattis and Nicholson's remarks come just days after the Taliban pulled off the single-most deadly attack against Afghan security forces since the beginning of the war.
Roughly a dozen militants infiltrated a sprawling Afghan base near the city of Mazar-i-Sharif. Using suicide vests and small arms, the militants - disguised as Afghan soldiers - wreaked havoc at the installation and, according to some reports, killed at least 140 Afghans and wounded 60.
The six-hour assault began as Afghan soldiers were leaving their weekly prayers or ambling to the base's dining facility. The Taliban fighters were eventually killed by a response force led by Afghan commandos. Nicholson praised the elite but overworked unit's response for bringing the "atrocity to an end."
It is unclear what the attack will do to Afghan forces' recruitment efforts, already strained by high casualties and low retention rates among the ranks. The Taliban has pledged that the attack is just the beginning of its annual spring offensive. However, since US combat troops mostly withdrew in 2014, the pace of Taliban attacks has remained consistent across the country year-round.
Currently, there are 8400 US troops in Afghanistan split between performing two roles. One contingent helps advise the Afghan security forces while the other carries out unilateral and partnered counterterrorism operations against groups such as Isis and al-Qaeda. In addition, there are roughly 5000 Nato troops in Afghanistan split between various areas of responsibility.
Nicholson and the chief of US Central Command, Army, General Joseph Votel, have both agreed that roughly 3000 additional troops are needed to help prop up the Afghan security forces and break what top US officials have called a "stalemate" in the country. At the height of the war there were more than 100,000 troops in the country.
Mattis said that he is still deciding whether he'll ask President Trump to send more troops.
