Russia has issued an ultimatum to Ukrainian forces in Crimea to clear out by 5 a.m. Tuesday (10 p.m. EST on Monday) or face a "military storm," Russia's state-run news agency Interfax reported.
In full, the statement read, "If they do not surrender before 5 a.m. (Ukrainian time) tomorrow, a military storm will commence against units and divisions of the armed forces across Crimea."
"The ultimatum is to recognize the new Crimean authorities, lay down your weapons and leave, or be ready for an assault," said Vladyslav Seleznyov, the regional ministry spokesman for the Crimea. He said base commanders had informed the ministry of "different times" for the ultimatum to expire.
The Russian Black Sea fleet based in Crimea denied Monday there were plans to storm Ukrainian military positions on the peninsula, calling reports of an ultimatum "nonsense," Interfax said.
"That is complete nonsense," a representative of the fleet was quoted as saying after Ukraine's regional military said it had received an ultimatum to surrender early Tuesday or face attack.
"We are used to daily accusations about using force against our Ukrainian colleagues," he said. "Efforts to make us clash won't work."
Ukraine says Russian forces controlling the strategic region of Crimea are demanding that the crew of two Ukrainian warships must surrender.
Ukrainian Defense Ministry spokesman Maksim Prauta said four Russian navy ships were blocking Ukraine's anti-submarine warship Ternopil and the command ship Slavutych in Sevastopol's harbor. He said the Russians ordered the crew to surrender within the hour or face Russians storming and seizing the ships and crew.
U.S. Vice President Joe Biden called Russian Prime Minister Dmitry Medvedev on Monday, and called on Moscow to pull back forces in Crimea, the White House said.
Officials noted that the call took place after Medevedev spoke over the weekend to Ukraine's new prime minister Arseniy Yatsenyuk, but did not give further details.
"The vice president urged Russia to pull back its forces, support the immediate deployment of international monitors to Ukraine, and begin a meaningful political dialogue with the Ukrainian government," a White House statement said.
Monday's call came as Washington mulled its options on how to respond to Moscow's incursion into Ukraine and after a tense 90-minute telephone call between President Barack Obama and Russian President Vladimir Putin on Saturday.
European Union governments agreed on Monday to consider "targeted measures" against Russia if Moscow fails to de-escalate the crisis in Ukraine, an EU diplomat said.
In a statement agreed at emergency talks on the issue, EU foreign ministers said: "In the absence of de-escalation steps by Russia, the EU should decide on consequences for bilateral relations ... And will consider about possible targeted measures", the diplomat said, speaking on condition of anonymity.
Global stock markets are down sharply on tensions over Russia's military advance into Ukraine and the threat of sanctions by Western governments.
The Dow Jones industrial average fell 202 points, or 1.2 percent, to 16,118 as of midday Monday.
The Standard & Poor's 500 index fell 20 points, or 1.1 percent, to 1,839. The S&P 500 closed at a record high Friday.
The Nasdaq composite fell 54 points, or 1.3 percent, to 4,253.
Russia's benchmark stock index plunged 12 percent. Stocks were also sharply lower in Europe. Germany's DAX fell the most, 3.4 percent.
Gold and bond prices rose as investors shifted money into safer assets. The price of crude oil rose 2 percent to $104 a barrel as traders worried about disruptions of Russian oil exports.