By Catherine E. Shoichet, Marie-Louise Gumuchian and Susanna Capelouto, CNN updated 6:46 PM EST, Mon March 3, 2014
CNN) -- Russia showed no signs of backing down Monday even as world leaders threatened sanctions and sternly rebuked the country for sending troops into Ukraine.
At an emergency U.N. Security Council meeting to discuss the unfolding crisis, Ukraine's envoy asked for help, saying that Russia had used planes, boats and helicopters to flood the Ukrainian peninsula of Crimea with 16,000 troops in the past week.
"So far, Ukrainian armed forces have exercised restraint and refrained from active resistance to the aggression, but they are in full operational readiness," Ukrainian Ambassador Yuriy Sergeyev said.
As diplomats at the meeting asked Russia to withdraw its troops and called for mediation to end the crisis, Russian Ambassador Vitaly Churkin insisted his country's aims were preserving democracy, protecting millions of Russians in Ukraine and stopping radical extremists.
He said ousted President Viktor Yanukovych remains Ukraine's elected leader and has asked Russia to send troops.
The Russian envoy read a letter from Yanukovych at the U.N. meeting, describing Ukraine as a country "on the brink of civil war," plagued by "chaos and anarchy."
"People are being persecuted for language and political reasons," the letter said. "So in this regard, I would call on the President of Russia, Mr. Putin, asking him to use the armed forces of the Russian Federation to establish legitimacy, peace, law and order, stability and defending the people of Ukraine."
U.S. Ambassador Samantha Power said Russia's claims about the situation in Ukraine are untrue and warned that sending military forces "could be devastating."
Yanukovych, she said, abandoned his post last month and was then voted out of office by Ukraine's democratically elected parliament.
"Russian military action is not a