Russia and the U.S. are increasingly sparring over Afghanistan, adding to rapidly souring ties between the Kremlin and President Donald Trump’s administration.

Defense Secretary James Mattis has voiced alarm at Russia’s actions in Afghanistan, where it’s been cultivating links with the Taliban amid a campaign waged by the terrorist group against Afghan and NATO forces.

His comments come as local Afghan officials and a former Taliban commander say there is evidence Russia is supplying arms to the insurgents. U.S. officials won’t go that far in public, but U.S. Central Command chief Gen. Joseph Votel has told a congressional panel that Russia was probably providing the group with weapons.

Moscow’s support for the Taliban risks adding another front to tensions with Washington after Trump last week ordered a missile strike on an airbase in Syria. The frictions are set to loom large over the meeting of Group of Seven foreign ministers in Italy, after which Secretary of State Rex Tillerson is due to travel to Moscow.

It also highlights the task for Afghan President Ashraf Ghani as he struggles to contain the Taliban amid a deteriorating security situation. The Taliban controls or contests over half the country’s populated areas, according to U.S. government estimates, making it harder for America to extract itself from its longest-ever war. Ghani has repeatedly called on the Taliban to join peace talks.

The Kremlin denies arming the group. Russia’s special envoy for Afghanistan, Zamir Kabulov said in a March 30 interview the accusations were an invention by the Afghan government and its allies to “justify their own failure on the battlefield.”

Still, Russia, loath to allow another permanent U.S. base near its borders, says it supports the Taliban’s demand for foreign troops to withdraw from Afghanistan. And it has accused the U.S. of sabotaging its efforts to help end the conflict by staying away from an April 14 peace initiative in Moscow.

“Russia is actively building contacts to have levers of influence in case the situation enters a crisis phase,” Petr Topychkanov, an analyst at the Carnegie Moscow Center, said. “That’s viewed very negatively in the U.S., which is the main sponsor in Afghanistan.”

Afghanistan is seen as a strategic launchpad for building influence over Central Asia’s former Soviet republics and its collapse could trigger a new round of regional instability as India, Pakistan, Iran and Russia step in to support government and insurgent groups. Apart from its geopolitical importance, the war-torn country owns highly rich, untapped mineral resources such as copper and lithium estimated by the Afghanistan Geological Survey to be worth around $3 trillion.

Trump promised during his election campaign to mend ties with Vladimir Putin. But the relationship quickly deteriorated as the new administration vowed to keep sanctions on Russia imposed over the Ukraine conflict and criticized Moscow’s actions in Syria and Libya.

Russia has denounced the U.S. air strikes on Syria, which the U.S. launched after blaming the regime of Syrian President Bashar Assad for a chemical weapons attack that killed more than 80 people.

After its 1979 invasion turned into a 10 year war that helped speed the collapse of the Soviet Union, Russia has for years branded the U.S. effort in the central Asian nation a failure. By offering support to its former enemy the Taliban, Russia is seeking to exploit the insurgency and displace American influence.

Officials in Moscow disclosed at the end of last year they’ve been in contact with the Taliban, which ruled Afghanistan from 1996 to 2001 until being overthrown in the U.S.-led invasion. Russia says it has common interests with the Taliban because the group is fighting Islamic State, which has claimed responsibility for a number of deadly attacks inside Russia.

Inside Afghanistan, the Russian role is viewed with suspicion. Moscow “must help the Afghan army fight international terrorism instead of helping the Taliban,” Defense Ministry spokesman Dawlat Waziri said. He could not confirm if Russia is providing aid to the insurgents.

The Taliban receives small arms, rocket launchers, cash and ammunition from Russia via Tajikistan, said Nasruddin Saeedi, district governor of Dasht-e Archi in northern Kunduz province, which shares a porous border with the former Soviet republic.

Taliban spokesman Zabihullah Mujahed denied Russia was giving military support, though he said the group did establish links with Russia about nine months ago.

“Our political office in Qatar held a few low-profile meetings, particularly with Russian ambassadors,” Mujahed said by phone. “The talks took place during international conferences in countries outside Russia.”

Still, former Taliban commander Khan Mohammad Cherik, who recently joined the government’s peace process with 200 of his soldiers, said Russia was providing military and financial help. “Russian weapons were brought to us through many channels in the last one-to-two years,” he said by phone.

While there is no proof of weapons being provided, “we have seen Russian activity vis-a-vis the Taliban,” Mattis said at a briefing in London on March 31. “What they’re up to there in light of their other activities gives us concern.”

Around 13,000 U.S. and NATO troops are in Afghanistan and the top U.S. commander is pushing for several thousand more. The U.S. estimates that only 57 percent of Afghanistan is under government control, a 15 percent decrease since November 2015.

Russia, which until 2015 had offered support to U.S.-led forces fighting the Taliban by letting military equipment transit its territory, has “practically cut itself off from the coalition,” Omar Nessar, head of the Moscow-based Center for Contemporary Afghan Studies, said.

Further challenging American leadership, Russia invited China, India, Pakistan and Iran, plus five ex-Soviet Central Asian states, the U.S. and Afghanistan to the April 14 meeting which it says can pave the way for talks between the Taliban and Afghan government. Moscow has said it is willing to host such a meeting.

The risk is Russia’s support of the Taliban “will backfire and help embolden extremist groups whose ultimate objective is to subjugate Afghanistan and then pose a risk to neighboring countries,” Omar Samad, a former Afghan ambassador to Canada and France, said. “Russia is misjudging the situation by bypassing Kabul and trying to sideline Washington.”