Washington has accused Moscow of stockpiling weapons for pro-Russian separatists in eastern Ukraine. Earlier, Kyiv declared a week-long unilateral cease-fire with the rebels.
The US State Department said on Friday that Moscow was readying tanks and artillery pieces for delivery to pro-Russian separatists in eastern Ukraine.
"We also have information that Russia has accumulated artillery at a deployment site in southwest Russia, including a type of artillery utilized by Ukrainian forces but no longer in Russia's active forces," said State Department spokeswoman Jen Psaki.
She added that "Russia may soon provide this equipment to separatist fighters." Earlier in the month, both Ukrainian and US officials accused Moscow of delivering surplus T-64 tanks and BM-21 multiple-rocket launchers to the pro-Russian separatists.
Ukrainian President Petro Poroshenko declared a week-long cease-fire on Friday. But the leader of the separatist Luhansk People's Republic rejected the proposed pause in fighting, according to the Interfax-Ukraine news agency.
"No one will lay down their arms until a full troop withdrawal from our land," Valeriy Bolotov said.
The Kremlin criticized the cease-fire, saying that "this is not an invitation to peace and negotiations, but an ultimatum to militias in the southeast of Ukraine to lay down their arms."
Russian troop movements
On Thursday, NATO Secretary-General Anders Fogh Rasmussen accused Russia of deploying several thousand additional troops to the Ukrainian border , calling the move a "very regrettable step backward."
The Kremlin said that the troop movement was routine and NATO member states had been informed beforehand.
But State Department spokeswoman Psaki said on Friday that "this is the closest Russian troops have come to the Ukrainian territory since their invasion of Crimea." Moscow annexed the Black Sea peninsula last March.
Threat of more sanctions
Meanwhile, Washington has imposed targeted sanctions against seven more pro-Russian separatists in eastern Ukraine. The seven individuals include separatist leaders in Donetsk, Slovyansk, Luhansk, and the Crimean city of Sevastopol.
In a telephone conversation on Friday, French President Francois Hollande and US President Barack Obama welcomed Kyiv's 14-point peace plan. They called on Russia to convince the separatists to lay down their arms and to prevent the transfer of materials across its border with Ukraine.
The two Western leaders warned that if Moscow did not play a constructive role in de-escalating tensions in eastern Ukraine, then further measures would be taken against Russia. That presumably refers to additional sanctions.
slk/jm (AP, AFP, Reuters)