Russia vetoed a Western-backed resolution condemning the Crimea referendum at a UN Security Council emergency vote Saturday but China abstained, isolating Moscow further on the Ukraine crisis.
The draft resolution, which says Sunday's referendum would have no validity, got 13 votes in the 15-member council. But it was rejected when permanent member Russia exercised its veto.
"Russia, isolated, alone and wrong, blocked the resolution's passage," US ambassador Samantha Power told the council at its seventh emergency session on Ukraine since the crisis began.
"This is a sad and remarkable moment," she said.
"As we speak, Russian armed forces are massing across Ukraine's eastern border," she added in a short speech.
China often backs Russia at the council, especially on Syria-related votes, and Western diplomats had seen its abstention as the best possible outcome from Saturday's vote.
Chinese ambassador Liu Jieyi said that passing a resolution on Ukraine at this moment would "only result in confrontation and further complicate the situation."
"China has always been fair and objective. We will continue to mediate and promote dialogue so as to play a constructive role in seeking a political solution to the crisis," he said.
He called for an international coordination mechanism to explore as soon as possible a political settlement, all parties to refrain from escalation, and for international financial institutions to help shore up economic stability in Ukraine.
Saturday's emergency meeting was called at Washington's request and the resolution had been drafted by the United States in very measured terms so that it could be accepted by Beijing.
Beijing has long defended the need to respect territorial integrity and does not back interference in the internal affairs of other countries.
When the Security Council ruled on a similar international crisis, between Russia and Georgia in 2008, Beijing abstained.
Russia's veto had been certain after last-ditch talks between US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov broke down in London on Friday.
"It is a secret to no one that the Russian Federation will vote against the resolution," Russian ambassador Vitaly Churkin told the council in opening remarks before the vote.
He defended Sunday's referendum as necessary to fill the "legal vacuum" that arose "as a result of an unconstitutional coup d'etat in Ukraine."
The resolution declared that the referendum on Crimea coming under Kremlin rule has "no validity and cannot form the basis for any alteration of the status of Crimea."
The draft resolution did not explicitly call for Russian troop reinforcements to withdraw from Ukraine's Crimean peninsula, where Moscow has military bases. Nor did it threaten sanctions.
British ambassador Mark Lyall Grant demanded Russia rethink its moves and work to find a peaceful solution.
"If this referendum takes place tomorrow it will have no validity, no credibility and no recognition. We trust that Russia will take notice of its isolation," he told the council.
"If Russia fails to respond to Ukraine's outstretched hand, it will lead to further escalation of tension in the region and further consequences for Russia," he said.
The resolution called on states to refrain from recognizing the result and from "any action or dealing that might be interpreted as recognizing any such altered status."
It was a convoluted formula to demand that Russia not annex Crimea.
The resolution also reaffirmed commitment to Ukraine's sovereignty and territorial integrity.
It urged Moscow and Kiev to hold direct talks and exercise restraint, and noted the willingness expressed by Kiev to protect the rights of all Ukrainians, including minorities.
Moscow says Russian speakers in eastern Ukraine have been threatened since the fall of pro-Moscow president Viktor Yanukovych after deadly protests late last month.