MOSCOW-The US State Department today accused Russia of quietly sending tanks and other heavy weaponry to separatist fighters in east Ukraine.

"We are highly concerned by new Russian efforts to support the separatists," deputy spokeswoman Marie Harf told reporters.

"In the last three days a convoy of three T-64 tanks, several BM-21 or Grad multiple rocket launchers and other military vehicles crossed from Russia into Ukraine," she said. "This is unacceptable."

Those old tanks are Cold War relics which are no longer used by the Russian military, though Ukraine's army still uses them and the Russians are said to have others in storage.

The US has long accused Russia of meddling in Ukraine and of providing arms and support to the separatist fighters. Friday's accusation, however, would mark a steep increase in that alleged assistance. The State Department said Russia is attempting to quietly arm pro-Russian separatist fighters with weapons that they could deny had come from Russia.

"We have information that Russia has accumulated tanks of a type no longer used by Russian forces at a deployment site in southwest Russia, and some of these tanks recently departed," the State Department added in a later statement.

"Russia will claim these tanks were taken from Ukrainian forces, but no Ukrainian tank units have been operating in that area," the statement said. "We are confident that these tanks came from Russia."

The sudden and mysterious appearance of tanks in rebel hands immediately raised suspicions early this week. Yesterday, Ukrainian officials claimed they had recently crossed the border from Russia. Russia has yet to comment on the matter.

Given the amount of disinformation on both sides of the border, however, claims about the tanks' origin had been treated cautiously. When asked about the reports on Thursday the State Department was unable to confirm them.

Today, Harf, the spokeswoman, dismissed claims that the tanks had been captured from Ukrainian military units.

"They were somehow pulled out of the Russian warehouses, someone taught them how to use them, and they were sent from Russia to Ukraine," she said.

The New York Times reported that intelligence about the tanks crossing had been shared with NATO allies on Friday and that Secretary of State John Kerry complained about them in a phone call with Russian Foreign Minister Sergei Lavrov on Thursday.

Ukraine's new president phoned Putin yesterday to complain about the weapons flowing in, according to Ukrainian news reports, but the Kremlin has yet to address the subject.

Yesterday, before they could confirm the crossing, the State Department would not say if this would lead to more US sanctions, which President Obama has threatened if Russia continues to destabilize Ukraine.