"The international community will not recognize the results of a poll administered under threats of violence," it said in a statement. "Russia's actions are dangerous and destabilizing."
Russia raised the stakes Saturday when its forces, backed by helicopter gunships and armored vehicles, took control of the Ukrainian village of Strilkove and a key natural gas distribution plant nearby — the first Russian military move into Ukraine beyond the Crimean peninsula of 2 million people.
The Russian forces later returned the village but kept control of the gas plant. On Sunday, Ukrainian soldiers were digging trenches and erecting barricades between the village and the gas plant.
"We will not let them advance further into Ukrainian territory," said Serhiy Kuz, commander of a Ukrainian paratrooper battalion.
Despite the threat of sanctions, Putin has vigorously resisted calls to pull back in Crimea. At the United Nations on Saturday, Russia vetoed a Security Council resolution declaring the referendum illegal.
However, Putin spoke with President Barack Obama and supported a proposal from Germany to expand an international observer mission in Ukraine, the Kremlin said Sunday in a statement after the vote.
"The heads of state noted that despite the differences in their assessments, it was necessary to work together to find a way to stabilize the situation in Ukraine," the statement said.
German Chancellor Angela Merkel, who also spoke with Putin on Sunday, wants more observers sent to tense areas, particularly in eastern Ukraine, her spokesman said. Putin told Obama that such a mission would be welcome but would need to be extended to all regions in Ukraine, the Kremlin said.
In Donetsk, one of the main cities in eastern Ukraine, pro-Russia demonstrators called Sunday for a referendum similar to the one in Crimea.
In Sevastopol, the Crimean port where Russia now leases a major naval base from Ukraine for $98 million a year, speakers blared the city anthem up and down the streets, but the military threat was not far away — a Russian naval warship still blocked the port's outlet to the Black Sea, trapping Ukrainian boats.
At a polling station inside a historic school, tears came to Vladimir Lozovoy, a 75-year-old retired Soviet naval officer, as he talked about his vote.
"I want to cry. I have finally returned to my motherland. It is an incredible feeling. This is the thing I have been waiting for for 23 years," he said.
But Crimea's large Muslim Tatar minority — whose families had been forcibly removed from their homeland and sent to Central Asia during Soviet times — remained defiant.
The Crimea referendum "is a clown show, a circus," Tatar activist Refat Chubarov said on Crimea's Tatar television station. "This is a tragedy, an illegitimate government with armed forces from another country."
The fate of Ukrainian soldiers trapped in their Crimean bases by pro-Russian forces was still uncertain. But Ukraine's acting defense minister, Igor Tenyuk, was quoted as saying Sunday that an agreement had been reached with Russia not to block Ukrainian soldiers in Crimea through Friday. It was not clear exactly what that meant.
Ethnic Ukrainians interviewed outside the Ukrainian Orthodox cathedral of Vladimir and Olga said they refused to take part in the referendum, calling it an illegal charade stage-managed by Moscow. Some said they were scared of the potential for widespread harassment.
Vasyl Ovcharuk, a retired gas pipe layer, predicted dark days ahead for Crimea.
"This will end up in military action, in which peaceful people will suffer. And that means everybody," he said. "Shells and bullets are blind."
Associated Press writers Dalton Bennett in Sevastopol, Yuras Karmanau in Strilkove, Jim Heintz and Maria Danilova in Kiev and David Melendy in Washington contributed to this report.
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