More than 15,000 Russian troops have amassed along the border with Ukraine as the Pentagon said Friday it appears Moscow is about to transfer “heavy caliber multiple rocket systems” to pro-Russia separatists.

Reuters cited the U.S. ambassador to NATO in a tweet reporting that 15,000 Russian troops are on the Ukrainian border. The news wire service didn’t offer further details about the deployment.

The development came as the Pentagon said a transfer of “heavy caliber multiple rocket systems” from Russia to pro-Russia rebels in Ukraine was “imminent.”

Russia is also reportedly arming the rebels with tanks, armored vehicles and other high-powered weapons, according to U.S. officials who spoke to the New York Times. Andriy Lysenko, a Ukrainian military spokesman, also told the paper that government forces were taking more fire from the Russian side of the border. The spokesman said Ukraine responded by shooting down three Russian drones.

“We have facts of shelling of Ukrainian positions from the territory of Russian Federation,” he said. “We have facts on the violation of air border between Ukraine and Russia.”

Russia’s alleged direct involvement in the Ukraine crisis has come under intense scrutiny, especially after the crash of MH17 last week in eastern Ukraine. Ukrainian and western intelligence sources said there is strong evidence that the Kuala Lumpur-bound Malaysia Airlines flight from Amsterdam was shot down using a Russian-made air-to-surface missile system they claimed was used by pro-Russia separatists.