Russia has vetoed a U.N. Security Council resolution that would have urged countries not to recognize the results of a Sunday referendum in Ukraine's Crimea region.
Russia was the only Security Council member to vote against the measure on Saturday, while China abstained. The 13 other council members backed the measure.
The resolution would have affirmed Ukraine's territorial integrity by declaring Sunday's Crimea referendum could "have no validity."
China's abstention could indicate further isolation for Moscow in its support of the Crimean referendum.
U.S. Ambassador Samantha Power reacted to the measure's defeat, saying "This is a sad and remarkable moment."
"The truth is that this resolution should not have been controversial. It was grounded in principals that provide the foundation for international stability and law - Article 2 of the U.N. Charter - the prohibition on the use of force to acquire territory and respect for sovereignty, independence, unity and territorial integrity of member states. These are principals that Russia agrees with and defends vigorously all around the world -- except, it seems, in circumstances that involve Russia."
Russian Ambassador to the U.N. Vitaly Churkin said the resolution ran counter to the principles of international law.
"We can not go along with its basic assumption, that is declaring illegal the 16 March planned referendum where the very people of the Crimean republic will determine their future."
China's U.N. Ambassador Liu Jieyi explained his country's vote.
"The drafting of the draft resolution by the Security Council, at this juncture, will only result in confrontation and further complicate the situation."
As Security Council members reacted to the vote's outcome, Ukrainian envoy Yuriy Sergeyev said Russian forces had moved into a Ukrainian region near Crimea.
"While were we discussing today the crisis in Ukraine, in Crimea, I got a call, 40 minutes ago - you noticed that somebody called me. Russian troops entered the mainland on the south from Crimea. Now we are facing a new development and we are to face the further danger [dangerous] stance of the Russian Federation. And I appeal to all of you to find means and measures in the Security Council and United Nations as well as on your bilateral level to do utmost possible to stop the aggressor."
Ukraine's defense ministry said its forces "repelled" the Russian troops.
Sunday's referendum in Crimea gives residents two choices: join Russia or support a significant strengthening of Crimea's autonomy within Ukraine.
A series of protests took place a day ahead of the vote.
A large pro-Russian demonstration turned violent in the eastern city of Donetsk on Saturday. Some protesters pushed their way past police who were guarding the entrance to the Uranian Security Service Building. The protesters kicked in glass windows and doors before entering the building where they caused further damage.
Meanwhile, hundreds of people turned out for a rally in Kyiv where they voiced support for Crimea remaining a part of Ukraine.
In Moscow, tens of thousands of demonstrators gathered for rival rallies on Crimea.
Supporters of the Crimea referendum waved Russian and Soviet Union flags as they marched to Moscow's Revolution Square. Many of them wore identical red and black outfits.
Separately, chanting opponents waved Ukrainian and Russian flags. Some voiced concern that Russian intervention in Crimea could lead to war.
International monitors began arriving in Crimea on Saturday to observe Sunday's referendum vote. The monitors were invited by Russia and pro-Russian Crimean officials.
International monitor Alexander Simov, a Bulgarian journalist, said he believes the vote is legitimate.
"I am going to watch the whole process. To see... I think it that it is a legal referendum. So I think it is going to be very legal."
"Western politicians cannot understand what is going on. Because as far as I understand these countries and especially the U.S. have a very weak informational base about what is going on in Russia first and secondly what is happening in Ukraine. And also western politicians of EU, they do not really understand what happens in Russia. Western countries are at a dead end. In fact, they cannot take any measures against Russia in this situation. And Russia is in control totally and securely."