WASHINGTON ?-- The US Defense Department on Monday evening said that it is suspending all military-to-military engagements with Russia because of Moscow's military movement in Ukraine.

The suspension covers bilateral meetings, port visits and planning conferences, according to Pentagon spokesman John Kirby, who added that the Pentagon values the relationship it has developed with the Russian military in the last few years.

The United States, he said, calls on Russia to de-escalate the crisis in Ukraine and to pull Russian forces in Crimea back to their bases "as required under the agreements governing the Russia Black Sea Fleet."

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The spokesman also stressed that the military has not changed its posture in Europe or the Mediterranean Sea.

Meanwhile, a spokesman for the Trade Representative was quoted by media as saying that the United States has also put trade and investment talks with Russia on hold over the Ukraine crisis.

The moves came after President Barack Obama said his administration was examining "a whole series of" economic and diplomatic steps to "isolate" Russia over its refusal to withdraw military forces from Crimea.

Washington and its allies have already decided to suspend preparations for the Group of Eight summit slated for June in Russia's Black Sea resort of Sochi.

Crimea, a multiethnic region in southeastern Ukraine with a large Russian population, enjoys a high degree of autonomy after Ukraine gained independence from the Soviet Union in 1991. Russia has maintained its only Black Sea naval base there in the port of Sevastopol.

Russian President Vladimir Putin has said that his country could not stay aside if violent actions were taken against Russian speakers in eastern Ukrainian regions and Crimea.

But he has also stressed that Russia would take necessary measures according to international law and that further escalation of the crisis in Ukraine should be averted.

On Monday, the Russian foreign ministry refuted allegations of the North Atlantic Treaty Organization that Russia was conducting military escalation in Ukraine's Republic of Crimea.

"Providing security for fleet facilities and preventing possible attacks on our compatriots from extremists and radicals are the sole purposes of all troop movements," the ministry said.

The ministry stressed that it was obvious that the sole source of the current threats to the sovereignty and territorial integrity of Ukraine was the internal political crisis in the country.

In the United Nations, Russian envoy Vitaly Churkin said ousted Ukrainian Viktor Yanukovych has requested Putin in a Saturday letter to use force to help restore law and order in his turmoil-stricken country.

Meanwhile, Sergei Naryshkin, speaker of the State Duma, or the lower house of Russia's parliament, said there was no need to send Russian troops to Ukraine so far and an opportunity to settle the situation in Ukraine via a political dialogue still exists.

Special: Ukraine crisis

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US suspends trade, investment talks with Russia over Ukraine