UPDATE, 8:10p ET: The Pentagon says the U.S. is putting all military-to-military engagements with Russia, including exercises and bilateral meetings, on hold in light of recent events in Ukraine.

UPDATE 6:10p ET: Ukraine claims 16,000 Russian troops have been deployed in the flashpoint peninsula of Crimea since last week.

UPDATE 1:00p ET: Russian officials are now denying that there has been a deadline set for an assault on Ukrainian forces in Crimea, according to Reuters.

In the clip above, watch the report from Sevastopol, in the Crimea region, by Jessica Golloher of Fox News Radio.

Amb. Power: 'Russian Mobilization Is a Response to an Imaginary Threat'

Russia's fleet has reportedly ordered Ukraine's forces in the disputed Crimea region to surrender by 5 a.m. local time or 'face a storm,' a Ukrainian defense ministry source says. 

Watch Amy Kellogg's report from Kiev on the latest developments.

Update, 12:15p ET: Dow Jones industrial average falls more than 200 points at midday as global markets react to Russian invasion of Ukraine and threat of sanctions by West.

Read more from FoxNews.com:

“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.

Ukraine’s defense ministry did not immediately confirm the statement, Reuters reports. The U.N. Security Council is scheduled to hold a meeting this afternoon to address the crisis.

The reported threat came hours after Ukraine’s new leaders called for Western nations to rally against Russia’s invasion of the country’s 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.

"Those who are trying to interpret the situation as a sort of aggression and threatening us with sanctions and boycotts, these are the same partners who have been consistently and vigorously encouraging the political powers close to them to declare ultimatums and renounce dialogue," Lavrov said. "We call upon them to show a responsibility and to set aside geopolitical calculations and put the interests of the Ukrainian people above all."

Lavrov will meet later Monday with U.N. Secretary General Ban Ki-moon to discuss the situation.