Russian President Vladimir Putin has said he supported Ukrainian President Petro Poroshenko's call for a ceasefire in the country.
Mr Putin added that without "practical actions" to start negotiations the plan would not be viable.
"Vladimir Putin supports the decision of the President of Ukraine Petro Poroshenko to cease fire in the southeast of Ukraine, as well his stated intentions to take a number of concrete steps to achieve a peaceful settlement," the Kremlin said in a statement.
"However, the Russian head of state drew attention to the fact that the proposed plan without practical actions aimed at the beginning of the negotiation process will not be viable and realistic."
Mr Putin earlier ordered central Russia forces on "combat alert", according to defence minister Sergey Shoygu.
It comes a day after the Kremlin confirmed it was beefing up its military presence at the border with Ukraine.
Meanwhile, the United States has blacklisted seven separatists in Ukraine and threatened "scalpel" sanctions on Russia's financial, defence 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.
President Barack Obama spoke separately yesterday with French President Francois Hollande and German Chancellor Angela Merkel, the White House said.
The three leaders agreed that Russia faces the risk of new sanctions from the West if it fails to defuse tensions on the Ukraine border, the White House added.
Separatist rebellions erupted in eastern Ukraine in early April after street protests in Kiev toppled the Moscow-backed leader Viktor Yanukovych 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 companies will be generally be 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 that 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.
A senior Obama administration official who spoke on condition of anonymity said Ukrainian officials 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."
The official also said the US had intensified its conversation with the EU about imposing additional targeted sanctions sanctions on Russia because of the flows of Russian material to Ukraine.