The United States blacklisted seven separatists in Ukraine on Friday and threatened "scalpel" sanctions on Russia's financial, defense and high-tech industries as more Russian military material has flowed into Ukraine.
The US moves respond to what American officials say is Russia's recent increase in support to Ukrainian separatists, including the provision of Russian tanks and the preparation of more to cross into eastern Ukraine.
Separatist rebellions erupted in eastern Ukraine in early April after street protests in Kiev toppled the Moscow-backed leader Viktor Yanukovich and Russia in turn annexed the Crimean peninsula. Eastern rebels have called for union with Russia.
The US Treasury named seven people, including separatist leaders in Donetsk, Slovyansk, Luhansk and the Crimean city of Sevastopol, whose assets under US jurisdiction will be frozen and with whom US individuals and firms will be generally barred from dealing.
The Treasury said one of the seven was a Russian citizen but gave no information on the citizenship of the other six.
Separately, the US State Department said it was confident Russia last week sent tanks and rocket launchers from a deployment site in southwest Russia into eastern Ukraine and believed more tanks were being readied for delivery.
"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, and believe Russia may soon provide this equipment to separatist fighters," said Jen Psaki, a spokeswoman for the State Department.
A senior Obama administration official who spoke on condition of anonymity said Ukranian officials today told diplomats in Brussels that they had "evidence of some additional 10 tanks, fuel trucks, and additional supporting vehicles coming over the border outside of Luhansk in the last 24 hours."
"What I am able to confirm from ... is that we have information that additional tanks departed from a deployment site in southwest Russia yesterday," said the official.
The official also said the United States had intensified its conversation with the European Union about imposing additional sectoral sanctions on Russia because of the flows of Russian materiel to Ukraine.
"We have been in active conversations with our EU partners on what we call 'scalpel' sanctions, which would be targeted primarily in the financial, defense and high technology sectors," the official told reporters.
"The idea here is to deny Russia the kinds of investment and next-generation technology that it needs to continue to grow," the official said.
"This conversation has been on going for some time but it has intensified over the last week as we have seen Russian materiel move into Ukraine," the official said. Those conversations are continuing today and over the weekend and next week Secretary (of State) Kerry will be making calls.