Ukraine's mission to the United Nations is claiming that 16,000 Russian troops have been deployed in the strategic Crimea region, while Russia's UN ambassador told the council Monday that Ukraine's fugitive president requested troops.

A letter from Ukraine's UN mission circulated during Monday's emergency meeting of the UN Security Council pleads for international help.

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NATO Secretary-General Anders Fogh Rasmussen has announced an emergency meeting on Ukraine for Tuesday, following a request from Poland. (Francois Lenoir/Reuters)

Russia's UN ambassador told the council that Ukraine's fugitive president requested Russia's armed forces to establish law and order in the former Soviet country.

The letter from ousted Ukrainian leader Viktor Yanukovych described Ukraine as being in a state of "outright terror and violence driven by the West," and asked that Russia intervene.

"I appeal to the President of Russia Vladimir V. Putin to use the Armed Forces of the Russian Federation to re-establish the rule of law, peace, order, stability and to protect the people of Ukraine," the letter read.

But Samantha Power, the U.S. ambassador, told an urgent meeting of the Security Council: "One might think that Moscow has just become the rapid response arm of the High Commissioner for Human Rights."

It was the third meeting of the Security Council since Friday, when Russian planes and troops started moving into Crimea.

The public meeting was meant to allow countries representing all areas of the world to express their views on the Ukraine crisis.

EU threatens sanctions

Meanwhile, European Union leaders called a special summit for Thursday, where they are expected to freeze visa liberalization and economic co-operation talks with Russia if Moscow hasn't taken steps to calm the crisis in Ukraine's Crimean peninsula.

EU foreign ministers said they also have stopped preparation for the G8 summit, which is set for June in the Russian resort of Sochi.

EU foreign policy chief Catherine Ashton said the EU would give Russia until Thursday to show clear signs of goodwill, including a willingness to open talks and a withdrawal of Russian troops to their barracks in Crimea.

"The ambition is to see the situation improve. If it doesn't, then the course is set," Ashton said after the foreign ministers' meeting.

She said she will meet Russian Foreign Minister Sergei Lavrov on Tuesday in Madrid.

The ambassadors of NATO's 28 member nations will hold a second emergency meeting on Ukraine Tuesday after Poland invoked an article calling for consultations when a nation sees its "territorial integrity, political independence or security threatened," the alliance said in a statement.

At the EU meeting, many foreign ministers stressed the immediate focus should be on diplomacy and promoting direct dialogue between Russia and the new leadership in Ukraine.

The EU is Russia's biggest trading partner, and Russia is the EU's third-largest partner, mostly thanks to exports of raw materials such as oil and gas.

Economic sanctions would hurt all sides, said Dutch Foreign Minister Frans Timmermans.

"Those consequences will be bad for everyone, but for Russia they will be far worse than for the EU. We can target other markets if we have to. [Russia] will have trouble to quickly find other customers," he said.