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The Moscow-backed government of Crimea set a referendum in 10 days to ratify its decision to secede from Ukraine and join Russia, dramatically escalating tension as the West tries to negotiate a withdrawal of Russian troops from the area.

The step Thursday came as Western diplomats were huddled in Rome with their Russian counterparts to end the political standoff, and just two days after President

Vladimir Putin

said Russia wasn’t interested in annexing Crimea.

U.S. and European leaders said such a referendum would violate the Ukrainian constitution and international law.

“Any discussion about the future of Ukraine must include the legitimate government of Ukraine,” President

Barack Obama

said at the White House. “In 2014, we are well beyond the days when borders can be redrawn over the heads of democratic leaders.”

A Russian move to absorb Crimea against the will of Ukraine’s national government in Kyiv would mark the first time since World War II that such a maneuver had been attempted in Europe.

“This is an illegitimate decision. This so-called referendum has no legal grounds at all,” said Ukraine’s Prime Minister Arseniy Yatsenyuk, who was in Brussels meeting with European leaders working to forge a response. “Crimea was, is, and will be an integral part of Ukraine.”

Shortly after the Crimean legislature’s decision, the White House imposed visa restrictions and laid the groundwork for potential sanctions on those who have worked to destabilize Ukraine.

At an emergency European Union summit in Brussels, leaders said that if Russia doesn’t begin negotiations with Ukraine within days and quickly produce results, the bloc would follow through with its own sanctions on Moscow, including travel bans and asset freezes.

“These last days have seen perhaps the most serious challenge to security on our continent since the Balkan wars,” said

Herman Van Rompuy,

president of the European Council, who chaired the summit. “We strongly condemn Russia’s unprovoked violation of Ukraine’s sovereignty and territorial integrity.”

Crimea has been under de facto military occupation since Feb. 27, when thousands of heavily armed men seized key locations on the peninsula and effectively cut it off from the rest of Ukraine. The men were wearing unmarked uniforms but were widely believed to be Russian soldiers, many from Russia’s Black Sea Fleet, which is based in the Crimean port of Sevastopol.

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The leader of Crimea’s Tatar minority held a press conference to call for a boycott of a referendum planned by the region’s recently installed pro-Russian leadership to decide whether Crimea should join the Russian Federation.

The gunmen took over the parliament and presided over the installation of a new pro-Russian governor for the region, who then announced a referendum would be held on the more vaguely described issue of increased autonomy from Kyiv.

It was initially set for May 25, the same day Kyiv’s new government has set to pick the country’s next president, but later moved forward to March 30, but now moved to March 16.

Crimea’s parliament said the decision to hold a referendum was “the result of the unconstitutional coup” that put a new government in place in Kyiv last month and the “flagrant violation of the laws of Ukraine” by nationalist forces since the ouster of former President

Viktor Yanukovych.

Local residents hold a Soviet flag as members of the Cossack militia guard the parliament building in Simferopol, the Crimean region’s capital.
Associated Press

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President Barack Obama delivers a statement on the situation in Ukraine at the White House Thursday.
REUTERS

Moscow has welcomed the new Crimean government’s appeal for military and financial assistance and supported the new governor, while refusing to recognize the new government in Kyiv.

The referendum announcement sent Russian shares down, and in afternoon trading Moscow’s Micex index was 2.4% lower, while the RTS index was down 3.2%.

Shortly after announcing the referendum, Crimea’s parliament voted separately in favor of joining Russia, but lawmakers said the final decision will rest with the people. The government also directed an appeal to Mr. Putin, asking if Russia would be prepared to absorb Crimea.

Earlier this week, Mr. Putin said Russia wasn’t looking to annex Crimea and said that “only the citizens themselves can determine their own future.” On Thursday, the Kremlin said Mr. Putin had discussed the possibility of Crimea becoming part of Russia during a meeting of his security council.

In Moscow, the Russian parliament was preparing to move up consideration of a draft law that would ease the annexation of new territories to Russia and officials said it could be passed in time for the referendum.

“This issue may be resolved within a very short time, like three to five days,”

Anatoly Lyskov,

a member of the Russia’s Federation Council’s constitutional legislation and judicial issues committee, told the Interfax new agency.

In a televised address in Kyiv, acting Ukrainian President

Oleksandr Turchynov

called the referendum “a farce and a crime against the state, organized by Russia.” He said that lawmakers in Kyiv would begin the procedure of dissolving the regional legislature in Simferopol. It wasn’t clear how those decisions would be enforced however, since the Crimean government no longer recognizes Kyiv’s authority.

The March 16 vote will ask voters whether they support “reunification with Russia” or if they agree to remain within Ukraine, keeping the substantial autonomy from the central government that the predominantly Russian-speaking region has had since 1992.

The parliament said that the decision will be made by a simple majority of voters.

Sergei Tsekov,

a legislator from Crimea’s Russian Unity party, said the parliamentary vote was taken in a closed session and about three-quarters of the representatives voted to join Russia. Asked why the session was closed, he said: “There was no need for observers because we’re all honest people in there.”

Mr. Tsekov, who is a member of the Russian Unity party in Crimea, said Ukraine “got a present when it was given Crimea” in 1954 by Soviet leader

Nikita Khrushchev.

“It wasn’t a present that anyone had the right to give. If we call things by their name,” he said, “then we should say it was Ukraine that annexed Crimea. I’m sure the population will applaud the decision that we made today to return to our brethren in Russia.”

Outside the parliament, about 200 demonstrators waved Russian flags after the vote. In the past week, Crimea’s parliament has debated changing the region’s time zone to Moscow time and adopting the Russian ruble if Crimea opts to secede.

Ukraine’s breakaway territory of Crimea said Thursday that it will hold a referendum on March 16 whether to formally secede from Ukraine and join the Russian Federation. WSJ’s Margaret Coker joins on the News Hub from Crimea. Photo: Getty

“I’m for Russia because of one thing: Putin,” said

Nataliya Arbatova,

a retired nurse who was born in Russia and moved to Crimea for her job in the 1970s. “He’s strong and can put our lives back in order.”

Other residents, however, reacted with shock and dismay, as the magnitude of the consequences sunk in.

“This is a farce. Who are these people to decide the course of my life and my children’s lives? It’s a scenario playing out like an African dictatorship,” said

Oleg Ilushkin,

a Russian-speaking father of two. He was born in Russia’s Donbas region and lived in Crimea for 35 years as an engineer for the state railroad.

The high commissioner on national minorities for the Organization for Security and Cooperation in Europe,

Astrid Thors,

said in Kyiv that there were concerns about whether a legitimate vote could be held given the time frame and the large presence of military personnel on the peninsula.

An observer group from the OSCE was blocked from entering Crimea Thursday, the organization said. “We have concerns and are not sure there will be an expression of free will,” she said.

—Naftali Bendavid and Carol E. Lee contributed to this article.

Write to Lukas I. Alpert at lukas.alpert@wsj.com and Margaret Coker at margaret.coker@wsj.com