Germany's foreign minister said should the Crimea independence referendum be followed by steps to absorb the region into Russia, the EU will be forced to consider a further, third stage of sanctions.
Frank-Walter Steinmeier told reporters that Russia had so far rebuffed all European proposals to defuse the standoff in the Crimea region of Ukraine.
He said European states over the weekend would draw up a list of Russians who will be subject to visa restrictions and asset freezes as part of initial sanctions already agreed within the EU.
Russia's top general spoke to the chairman of Nato's Military Committee by telephone today about the situation in Ukraine, the Interfax news agency reported, citing the Russian Defence Ministry.
General Valery Gerasimov discussed "the state of Russia-Nato relations and the current situation in Ukraine" with Nato General Knud Bartels, according to the report.
Ukraine's acting president said Russian forces are concentrated on the border "ready to invade" but that he believed international efforts could end Moscow's "aggression" and avert the risk of war.
Oleksander Turchinov told a local television channel that, when Russian forces took over the southern region of Crimea last week, other units were concentrated on Ukraine's eastern border "ready for an invasion of the territory of Ukraine at any moment".
"We are doing all we can to avoid war, whether in Crimea or in any other region of Ukraine," he said, adding that Ukraine's own armed forces were in a state of full combat readiness.
Turchinov said, "All of civilised humanity supports our country. All the leading countries of the world are on the side of Ukraine, and I am sure that this united effort in the international arena, bringing together all democratic countries, can still allow us to halt this aggression."
Foreign Secretary William Hague has called for Crimea's independence referendum to be abandoned and warned that European sanctions against Russia will be stepped up if there is no progress on the Ukraine crisis in the "next few days".
The US and the European Union will respond with a "serious series of steps" against Russia if a referendum in Ukraine's Crimea region goes ahead on Sunday, Secretary of State John Kerry warned.
Kerry told a congressional hearing he hoped to avoid such steps, which include sanctions, through discussions with his Russian counterpart, Sergei Lavrov, in London tomorrow.
He said, "If there is no sign of any capacity to be able to move forward and resolve this issue there will be a very serious series of steps in Europe and here with respect to the options that are available to us."
US Secretary of State John Kerry said there are "contingencies" in place in case Russian forces move farther east into Ukraine.
Although Kerry stressed he hopes to avoid that happening, he said there was "no telling that we can".
US Secretary of State John Kerry said tensions between Russia and the US over Ukraine "has the capacity" to affect their relationship with Moscow on Syria.
US Secretary of State John Kerry has warned there will be a "serious series of steps" from the US and Europe against Russia if a planned referendum on Ukraine's Crimea goes ahead.
Russian Foreign Minister Sergei Lavrov and U.S. Secretary of State John Kerry discussed proposals for resolving the crisis in Ukraine during a telephone conversation on Thursday, the Russian Foreign Ministry said.
Lavrov and Kerry are due to meet in London on Friday.
The UK Foreign Secretary William Hague said that UK and United States are working closely together to find solution to the crisis in Crimea.
US-Russia #Ukraine talks will take place in London tomorrow with UK support
Mr Hague has spoken to US Secretary of State John Kerry, ahead of Mr Kerry's talks with his Russian counterpart Sergei Lavrov.