Us offer assistance to ukraine
US imposes sanctions on 'Putin's bridge' to Crimea
The US Department of the Treasury added dozens of people and companies to the list.
Relations between Moscow and Washington have soured over the conflict in Ukraine.
Fighting in eastern Ukraine is at its worst level since a ceasefire was agreed last September.
A summit of EU leaders is meeting in Brussels.
NATO said Russia had increased its forces along the border and now had around 12,000 troops in the area.
The rebels show no sign of giving up their arms, but opening talks would be a big step on the road to peace.
Kiev says Moscow must take concrete steps, including withdrawing its troops from the country.
Organisers in the main region holding the makeshift vote on Sunday said nearly 90 percent had voted in favour.
As tensions in eastern Ukraine soared, international mediators took new peace proposals to Kiev.
The SBU security service said one military Mi-24 helicopter gunship had been shot down, killing one.
The European Union is also expected to add targets to its Russia sanctions list later on Monday.
The sources said Obama was expected to speak in a conference call on Friday with British Prime Minister.
Pro-Moscow separatist gunmen have shown no signs of surrendering govt buildings.
The standoff with Russian troops at the border has raised fears of a military confrontation with Moscow.
Tensions have risen in the mainly Russian-speaking east since the overthrow of Ukraine's president.
Ukraine’s security said separatists occupying the security headquarters had planted bombs in the building.
More than 100 people were killed, most by police snipers, on the streets of the capital.
Kerry says Washington still considers Russian actions in Crimea to be illegal and illegitimate.
Vladimir Putin aims to erect a Eurasian Union of former Soviet states