“We continue to get reports of this assistance,” Nicholson said, speaking to reporters alongside Defense Secretary Jim Mattis. “We support anyone who wants to help us advance the reconciliation process, but anyone who arms belligerents who perpetuate attacks like the one we saw two days ago in Mazar-e Sharif is not the best way forward to a peaceful reconciliation.”

A senior U.S. military official, who spoke on the condition of anonymity to discuss intelligence on the issue, said the Russians have increased their supply of equipment and small arms to the Taliban over the past 18 months. The official said the Russians have been sending weapons, including medium and heavy machine guns, to the Taliban under the guise that the materiel would be used to fight the Islamic State in eastern Afghanistan. Instead, the official said, the weapons were showing up in some of Afghanistan’s southern provinces, including Helmand and Kandahar — both areas with little Islamic State presence.

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“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 defense secretary. He added that it would have to be dealt with as such.

In the past, Nicholson has criticized 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 U.S. weaponry, such as shoulder-fired surface-to-air missiles. In March, when the commander of U.S. forces in Europe, Army Gen. 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 policy failures in Afghanistan.

Mattis and Nicholson’s remarks come just days after the Taliban pulled off the single deadliest attack against Afghan security forces since the beginning of the war.

On Friday, roughly a dozen militants infiltrated a sprawling Afghan base near the northern city of Mazar-e Sharif. Using suicide bomb 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.

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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 actions for bringing the “atrocity to an end.”

It is unclear how the attack will affect 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 U.S. combat troops mostly withdrew in 2014, the pace of Taliban attacks has remained consistent across the country year-round.

There are 8,400 U.S. troops in Afghanistan split between two roles. One contingent helps advise the Afghan security forces while the other carries out unilateral and partnered counterterrorism operations against groups such as the Islamic State and al-Qaeda. In addition to the U.S. troops, roughly 5,000 NATO troops are in Afghanistan, split among various areas of responsibility.

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Nicholson and the chief of U.S. Central Command, Army Gen. Joseph Votel, have agreed that roughly 3,000 additional troops are needed to help prop up the Afghan security forces and break what top U.S. officials have called a “stalemate” in the country. At the height of the war, more than 100,000 U.S. troops were in the country.

Mattis said Monday that he is still deciding whether to ask President Trump to send more troops.