Pro-Russian rebels paraded European monitors they are holding in eastern Ukraine on Sunday, freeing one but saying they had no plans to release another seven as the United States and Europe prepared new sanctions against Moscow.
US President Barack Obama called for the United States and Europe to join forces to impose stronger measures to restrain Moscow. The White House said it will add names on Monday of people close to President Vladimir Putin and firms they control to a list of Russians hit by sanctions over Ukraine, and also impose new restrictions on high tech exports.
The European Union is expected to follow suit by adding to its own list of targeted Russian people and firms, but Washington and Brussels have yet to reach agreement on wider measures designed to hurt the Russian economy more broadly.
In Donetsk, where pro-Russian rebels have proclaimed an independent "people's republic", armed fighters seized the headquarters of regional television and ordered it to start broadcasting a Russian state TV channel.
Speaking during a visit to Malaysia, Obama said restraining Russian President Vladimir Putin's ambitions in Ukraine would depend on the United States and its allies finding a unified position on tighter sanctions.
"We're going to be in a stronger position to deter Mr. Putin when he sees that the world is unified and the United States and Europe is unified rather than this is just a US-Russian conflict," Obama told reporters.
White House deputy national security adviser Tony Blinken said the new US measures would be imposed on Monday, mostly focused on adding to a list of those barred from travel to the United States and hit by asset freezes.
"We will be looking to designate people who are in (Putin's) inner circle, who have a significant impact on the Russian economy. We'll be looking to designate companies that they and other inner circle people control," Blinken told CBS television.
"We'll be looking at taking steps, as well, with regard to high-technology exports to their defence industry. All of this together is going to have an impact."
The stand-off over Ukraine, an ex-Soviet republic of about 45 million people, has dragged relations between Russia and the West to their lowest level since the end of the Cold War.
After Ukrainians overthrew a pro-Russian president, Putin overturned decades of international diplomacy last month by announcing the right to use military force on his neighbour's territory. He has seized and annexed Ukraine's Crimea peninsula and massed tens of thousands of troops on the frontier.
Heavily armed pro-Russian gunmen have seized buildings in towns and cities across eastern Ukraine. Kiev and its Western allies say the uprising is directed by Russian agents. Moscow denies it is involved and says the uprising is a spontaneous reaction to oppression of Russian speakers by Kiev.
An international agreement reached this month calls on rebels to vacate occupied buildings, but Obama said Russia had not "lifted a finger" to push its allies to comply.
"In fact, there's strong evidence that they've been encouraging the activities in eastern and southern Ukraine."