Russia pressed hard Monday for Ukrainian politicians to return to the Feb. 21 agreement that promised to create a new unity government which would rule until an early election no later than December.
But the proposal seemed to be a non-starter as diplomats met in Brussels, Kiev and Geneva and warnings about the dangers of Russia's military actions were issued from a host of European capitals.
On the ground, Russian troops controlled all Ukrainian border posts Monday in Crimea, as well as all military facilities and a key ferry terminal, cementing their stranglehold on the strategic Ukrainian peninsula.
They also demanded that the crew of two Ukrainian warships immediately surrender or be stormed and seized, according to Maksim Prauta, the Ukrainian defense ministry spokesman.
Four Russian navy ships in Sevastopol's harbor were blocking Ukraine's anti-submarine warship Ternopil and the command ship Slavutych, waiting for their answers, he said.
Earlier in the day, Russian Foreign Minister Sergey Lavrov, at U.N. meetings in Geneva, explained the reasoning behind Russia's military invasion of Crimea.
"This is a question of defending our citizens and compatriots, ensuring human rights, especially the right to life," he said.
There have been no reports, however, of any hostilities toward Russian-speakers in Ukraine during the country's four months of political upheaval.
Tension between Ukraine and Moscow rose sharply after President Viktor Yanukovych was pushed out by a protest movement made up of people who wanted closer ties with the European Union, more democracy and less corruption. Yanukovych fled to Russia last month after more than 80 demonstrators were killed - mostly by police - near Kiev's central square but insists he is still president.
Meanwhile, the Obama administration says any Russian threat to Ukraine's navy would be a "dangerous escalation" of an extremely tense situation .
The State Department says Monday Washington would hold Moscow directly accountable for such an escalation but did not elaborate on potential consequences. But department spokeswoman Jen Psaki adds that she could not confirm if Russia had in fact made such threats.
Earlier Monday, a Ukrainian military spokesman said Russia had issued an ultimatum to the crews of two Ukrainian warships in Crimea, demanding that they immediately surrender or be stormed and seized.
Secretary of State John Kerry is leaving for Ukraine late Monday and then will travel to France and Italy. He had planned to see his Russian counterpart in Paris, but Psaki said that meeting was no longer certain.