WASHINGTON (AP) — The Pentagon says it is suspending exercises and other activities with the Russian military, in light of Moscow’s military involvement in Ukraine.

A Pentagon spokesman, Navy Rear Adm. John Kirby, said Monday evening that the U.S. military has “put on hold” all military-to-military engagements, including bilateral meetings, port visits and planning conferences.

Kirby said the Pentagon values the relationship it has developed with the Russian military in the last few years, to reduce the risk of military miscalculation.

He said the U.S. calls on Russian forces in the Crimea region of Ukraine to return to their bases.

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, meanwhile, 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 in pro-Russian eastern Ukraine.