The UN Security Council met in an emergency session Saturday after Russia's parliament unanimously voted to send troops to Ukraine's Crimean region.
Yuriy Sergeyev, Ukraine's UN ambassador, asked Security Council members to stop the aggression against his country, adding that the number of Russian troops already inside Ukraine is "increasing every hour."
He told reporters after the meeting that Kyiv is willing to work with Moscow about its concerns, but must pull back its troops. "We are ready for consultations, but what we demand is all the troops to be withdrawn; immediately withdrawn, because they are illegally present there," he said.
Russia earlier rejected bilateral talks with Ukraine's new government.
Russian Ambassador Vitaly Churkin said Western governments were trying to "whip up" the situation, encouraging protesters who were angry about the previous government's decision not to sign an association agreement with the European Union.
On Russian troops, he told the council that President Vladimir Putin had asked parliament for the possibility of the use of force in Ukraine, but that Putin has not yet acted on its approval.
Moscow has not confirmed new troop deployments, saying only that current movements are in conformity with existing bilateral agreements to protect its naval base at Sevastopol in Crimea.
U.S. envoy Samantha Power urged that international monitors and observers -- including from Europe and the United Nations -- be quickly sent to Ukraine to report on the situation.
She echoed President Barack Obama's call for Russia's military to stand down and told reporters that Moscow's actions violate its commitment to protect Ukraine’s sovereignty and territorial integrity.
"Less than one week since the sun set on the Sochi Olympics, we are at a critical moment," she said. "The United States considers the current actions by Russia in Ukraine as unacceptable behavior for a G8 member."
The White House said it is currently suspending its preparations for the G8 summit which is planned for June in Sochi.
UN Secretary-General Ban Ki-moon said in taped remarks after speaking with President Putin by phone Saturday that "cool heads must prevail and dialogue must be the only tool in ending this crisis."
Council members called for a de-escalation of tensions and international mediation to end the crisis.