KIEV, Ukraine (AP) - Russian troops said to be 16,000 strong tightened their stranglehold on Ukraine's Crimean Peninsula Monday, openly defying the U.S. and the European Union and rattling world capitals and stock markets.

The West struggled to find a way to get Russia to back down, but with little beyond already threatened diplomatic and economic sanctions, global markets fell sharply over the prospect of violent upheaval in the heart of Europe.

For its part, Moscow reiterated its price for ending the crisis: restoration of a deal reached with the opposition less than two weeks ago to form a national unity government in Kiev that represents pro-Russian as well as Ukrainian interests, with new elections to be held by December.

Ukraine accused Russia of piracy for blocking two of the besieged country's warships and ordering them to surrender or be seized.

The U.S. originally estimated that 6,000 Russian troops were dispatched to Crimea, but Ukraine's mission to the United Nations said Monday that 16,000 had been deployed. That stoked fears that the Kremlin might carry out more land grabs.

U.S. Secretary of State John Kerry was headed to Kiev in an expression of support for Ukraine's sovereignty, and the EU threatened a raft of punitive measures as it called an emergency summit for Thursday.

But it was Russia that appeared to be driving the agenda. Foreign Minister Sergey Lavrov said at a U.N. Human Rights Council session in Geneva that Ukraine should return to an agreement signed last month by pro-Russian President Viktor Yanukovych - but not Moscow - to hold early elections and surrender some powers. Yanukovych fled the country after sealing the pact with the opposition and foreign ministers of France, Germany and Poland.

"Instead of a promised national unity government," Lavrov said of the fledgling new administration in Kiev, "a government of the victors has been created."

The latest flashpoint came when Ukrainians said Russian troops had issued an ultimatum for two of the besieged country's warships to surrender or be seized.

"I call on the leadership of the Russian Federation. Stop the aggression, stop the provocations, stop the piracy! These are crimes, and you will be called to account for them," said acting Ukrainian President Oleksandr Turchynov.