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.
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 decision to decline to follow Russia and veto the Security Council measure could indicate further isolation for Moscow for its support of the Crimean referendum.
After the vote, 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."
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."
The measure would have affirmed Ukraine's territorial integrity by declaring Sunday's Crimea referendum could "have no validity."
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 day ahead of the vote, tens of thousands of protesters gathered in Moscow 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.
Also, hundreds of people turned out for a rally in Kiev where they voiced support for Crimea remaining a part of Ukraine.
U.S. Secretary of State John Kerry says Russian acceptance of a Crimean referendum to break off from Ukraine and join Russia would be an illegal "backdoor annexation."
Kerry met with Russian Foreign Minister Sergei Lavrov for six hours in London Friday in an effort to defuse the tensions in Crimea.
If Sunday's vote passes and the Russian parliament ratifies it, Kerry said that would violate international law, and there will be consequences. He said this is not a threat against Russia but a matter of respecting international standards for annexation and independence.
Lavrov said in a separate news conference Friday that the talks with Kerry were useful, but the two have "no common vision" on Crimea. He said Russia will "respect the will of the Crimean people," and he criticized the threat of U.S. and European Union sanctions on Russia as "counterproductive."
U.S. President Barack Obama said Friday he still hopes for a diplomatic solution to the crisis. But Kerry said it is clear Russian President Vladimir Putin will not make any moves until after Sunday's referendum.
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 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."