The European Union has agreed to consider "targeted measures" against Russia if Moscow fails to de-escalate rising tensions in Ukraine, a diplomat reportedly said on the sidelines of an emergency meeting Monday.
The diplomat spoke to Reuters on condition of anonymity.
Ukraine said Monday that Russian forces controlling the disputed peninsula of Crimea are demanding that the crew of two Ukrainian warships must surrender – or face capture, but Russia denied claims of an impending assault.
EXPLAINER: How Crimea differs from the rest of Ukraine
Earlier, a Russian Navy commander ordered that all Ukrainian forces in the region must surrender by 5 a.m. local time Tuesday or face "a real assault."
The 28 EU foreign ministers held an emergency meeting Monday to discuss their response to the Ukrainian crisis. The EU is threatening to freeze visa liberalization and economic cooperation talks and boycott the G8 summit in Russia's Sochi if Moscow does not back off in Ukraine.
French Foreign Minister Laurent Fabius said the EU would give Russia until an emergency summit of EU leaders is held on Thursday to show clear signs of goodwill, including a willingness to open talks and a withdrawal of Russian troops to their barracks in the Crimea.
If not, Fabius said the EU would start implementing punitive measures.
The uncertainty of the situation roiled global financial markets Monday, as Russia continued to impose its military presence on Ukraine.
Ukrainian Defense Ministry spokesman Maksim Prauta said Monday that 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.
But Russia's Black Sea Fleet later told the Interfax news agency that it had no plans to launch an assault on Ukrainian military units in Crimea, according to Reuters.
"If they do not surrender before 5am tomorrow, a real assault will be started against units and divisions of the armed forces across Crimea," Russia's Black Sea Fleet Commander Alexander Vitko told the Interfax news agency Monday, Sky News reports.
U.N. Security Council is scheduled to hold a meeting this afternoon to address the crisis.
"These reports today of threats of force against Ukrainian military installations would, if true, in our view constitute a dangerous escalation of the situation for which we would hold Russia directly responsible,'' U.S. State Department spokeswoman Jen Psaki told reporters in a conference call.
The reported threats came hours after Ukraine's new leaders called for Western nations to rally against Russia's invasion of the Crimean Peninsula, making a plea for economic and political support as Moscow continued to be defiant.
Ukrainian Prime Minister Arseniy Yatsenyuk insisted that Crimea remains Ukrainian territory despite the presence of thousands of Russian troops who have secured control over the region without suffering any casualties or firing a shot.
"Any attempt of Russia to grab Crimea will have no success at all. Give us some time," he said at a news conference with British Foreign Secretary William Hague, who is visiting Kiev.
"For today, no military options [are] on the table," he said, adding that what they urgently need is an economic and political support.
"Real support. Tangible support. And we do believe that our Western partners will provide this support," he said.
Hague said on the BBC that Moscow would face "significant costs" for taking control of Crimea.
But Russian Foreign Minister Sergey Lavrov on Monday justified the use of Russian troops streaming into the neighboring Crimea region as a necessary protection for his country's citizens living there.
The use of Russian troops is necessary "until the normalization of the political situation" in Ukraine, Lavrov said at an opening of a month-long session of the U.N. Human Rights Council in Geneva.
"We are talking here about protection of our citizens and compatriots, about protection of the most fundamental of the human rights -- the right to live, and nothing more," Lavrov said.
Ukraine has accused Russia of a military invasion and has called on the Kremlin to withdraw its troops. Lavrov dismissed the criticism, and said that "information is coming in about preparations for new provocations that are being committed, including against the Russian Black Sea fleet," which is based in Crimea, a strategic peninsula now effectively under Russian control.
Lavrov will meet later Monday with U.N. Secretary General Ban Ki-moon to discuss the situation.
Lavrov called on Ukraine to return to the Feb. 21 agreement signed by pro-Russian President Viktor Yanukovych aimed at ending his country's three-month political crisis. He fled after signing an agreement with the opposition and foreign ministers of France, German and Poland to hold early elections this fall and surrender much of his powers. But opposition supporters kept pushing for his immediate dismissal.
Lavrov said Yanukovych kept up the agreement, but criticized the opposition, saying they "did nothing."
"The illegal arms have not been relinquished, the government buildings and streets of Kiev have not been completely freed, radicals maintain control of cities," Lavrov said. "Instead of a promised national unity government a 'government of the victors' has been created."
Russia has said China is largely "in agreement" with its response to Ukraine, according to Sky News.
Pro-Russian troops on Monday took over a ferry terminal in the city of Kerch, on the easternmost tip of the peninsula, approximately 12 miles by boat from Russian territory. The men refused to identify themselves, but they spoke Russian and the vehicles transporting them had Russian license plates.
Troops that Ukraine says are Russian soldiers have also occupied airports in Crimea, smashed equipment at an air base and besieged a Ukrainian infantry base.
Border guard spokesman Sergei Astakhov said the Russians were demanding that Ukrainian soldiers and guards transfer their allegiance to Crimea's new pro-Russian local government.
"The Russians are behaving very aggressively, they came in by breaking down doors, knocking out windows, cutting off every communication," he said.
He said four Russian military ships, 13 helicopters and 8 transport planes had arrived in Crimea in violation of agreements that permit Russian to keep its naval base at Sevastopol.
On Sunday night, Ukraine's defense ministry said two Russian fighter jets violated the country's airspace in the Black Sea and that it scrambled an interceptor aircraft in response, according to Sky News.
Western leaders were left scrambling for possible ways to defuse the crisis as phone calls were exchanged and threats and protests were made. Outrage over Russia's military moves mounted in world capitals, with U.S. Secretary of State John Kerry calling on President Vladimir Putin to pull back from "an incredible act of aggression." Kerry is heading to Kiev Tuesday for diplomatic talks.
NATO is urging Russia to pull back its forces and seek a peaceful resolution through dialogue with Ukraine.
Ukraine is not a NATO member, which means the United States and Europe are not obligated to come to its defense. But Ukraine has taken part in some alliance military exercises and contributed troops to its response force.
So far, however, Ukraine's new government and the West have been powerless to counter Russia's tactics.
Russia has long wanted to reclaim the lush Crimean Peninsula, part of its territory until 1954. Russia's Black Sea Fleet pays Ukraine millions annually to be stationed at the Crimean port of Sevastopol and nearly 60 percent of Crimea's residents identify themselves as Russian.
The Associated Press contributed to this report.