Ukranian military commander told ITV News the Russian men "have come to protect us. So why are their guns pointing towards our base?"
Fear is shaping events in Crimea, the multi-ethnic peninsula that finds itself at the heart of a tussle between great powers.
The Cold War has returned with a vengeance, judging by the mood in Washington with talk of extensive sanctions against Russia.
US ambassador to the UN Samantha Power wrote on Twitter:
The proposed referendum on the future of #Crimea would violate the Ukranian constitution. The U.S. will not recognize its results.
US Secretary of State John Kerry said he believes it is "imperative" to act quickly and find a political solution to the Ukraine crisis.
Kerry addressed reporters at the US embassy in Rome following a day of talks with EU ministers and Russian Foreign Minister Sergei Lavrov.
He said Lavrov was personally delivering their proposals to Putin.
"We have agreed to stay in close touch in order to see if there is a way forward to try to get to the negotiating table with the parties necessary to be able to stabilise this," he said, adding that acting quickly was key in resolving the crisis.
There were tense scenes in Ukraine's eastern city of Donetsk after several dozen pro-Russia demonstrators were detained.
President Barack Obama has urged Russian President Vladimir Putin to accept the terms of a potential diplomatic solution to the Ukraine crisis.
In their second phone conversation in the past six days, Obama outlined the terms of a diplomatic "off-ramp" that US officials are promoting.
Under terms of the deal, Russia would pull back troops to bases in Crimea, allow international monitors in to ensure the rights of ethnic Russians are respected and consent to direct talks with Ukraine officials.
"President Obama indicated that there is a way to resolve the situation diplomatically," the White House said in a statement.
US President Barack Obama spoke to Russian President Vladimir Putin for an hour to discuss the situation in Ukraine.
"President Obama emphasised that Russia’s actions are in violation of Ukraine’s sovereignty and territorial integrity, which has led us to take several steps in response, in coordination with our European partners," the White House said.
European Union leaders agreed minor steps in response to Russia's "illegal" intervention in Ukraine's Crimea region today.
David Cameron said the EU would consider even further steps against Russia if the situation persists after he dismissed the potential economic consequences of defending Ukraine.
President Obama said the resolve of the United States, its allies and the international community would "remain firm" if the "violation of international law" continued.
ITV News' International Affairs Editor Rageeh Omaar reports:
Barack Obama has condemned a move by Ukraine's Crimea region to set up a referendum to join Russia.
"The proposed referendum on the future of Crimea would violate the Ukrainian constitution and violate international law," the US President said.
Mr Obama said "any discussion about the future of Ukraine must include the legitimate government of Ukraine".
He added: "In 2014, we are well beyond the days when borders can be redrawn over the heads of democratic leaders."
US President Barack Obama said if the "violation of international law continues, the resolve of the United States, our allies and the international community will remain firm" in response to Russian actions in Ukraine.