Kiev, Ukraine (CNN) -- A Crimean parliament decision to hold a referendum on separation from Ukraine is an "illegitimate decision," interim Ukrainian Prime Minister Arseniy Yatsenyuk said Thursday.
"Crimea was, is, and will be an integral part of Ukraine," he said at a news conference.
Lawmakers in the Crimea region voted earlier in favor of leaving Ukraine for Russia, which already has the Black Sea peninsula under de facto control, and set a referendum on the move for 10 days' time.
Citizens of Crimea will face a simple choice: Stay in Ukraine or join Russia.
It's not clear how easily the region could split off if the referendum endorses the move.
Yatsenyuk, speaking at a news conference in Brussels, Belgium -- where EU leaders are meeting to discuss possible sanctions against Russia -- said the referendum "has no legal grounds at all."
"That's why we argue that the Russian government should not support those who support separatism."
He also insisted that his country would pursue a peaceful, diplomatic resolution to the crisis but would not be subordinate to Russia.
Ukraine's government will do nothing to jeopardize its country or its principles of territorial integrity, sovereignty and independence, he said.
"If they are ready to talk, we are -- and we made it very clear," he said of the Russians.
Justice Minister Pavlo Petrenko told journalists in Kiev that the Crimean parliament's decision is illegal because under the constitution, only national referendums are permitted, a Justice Ministry spokeswoman said.
The autonomous region has a 60% ethnic Russian population, having been part of Russia until it was ceded to Ukraine in 1954 by the Soviet Union.
But not everyone may be as keen on coming under Moscow's direct influence. A quarter of the peninsula's population is Ukrainian and about 12% Crimean Tatars, a predominantly Muslim group.
The parliament in Crimea installed a new, pro-Moscow government late last month -- as armed, pro-Russian men besieged the parliament building -- and does not recognize the authorities in Kiev. It had previously said a referendum on greater autonomy for Crimea would be held at the end of March.
Citizens will now be asked on March 16 if they want an autonomous republic of Crimea within Russia; or within Ukraine.
Ukrainian troops told to leave
Michael Crawford, a former British ambassador in Eastern Europe, cautioned that whatever the result of the vote, it may be meaningless.
"It does not follow that if Crimea votes to join Russia, that anyone will accept it," he said.
"For Russia to start cherry-picking bits of the former Soviet Union, cranking up referenda in Kazakhstan or Latvia or wherever you like, to try to carve off bits, would be against international law, and it would be something Vladimir Putin has said he doesn't want to do."
Putin, the Russian President, has insisted Russia has the right to use military force in Ukraine if necessary to protect ethnic Russians.
But he has denied claims by Ukrainian officials and Western diplomats that Russia has sent thousands of troops into the region in recent days. Russia says the heavily armed troops, in uniforms without insignia, are local "self-defense" forces.
The deputy speaker of the Crimean parliament, Rustam Temirgaliev, said Thursday at a news conference that the only forces allowed in Crimea are the Russian military -- and that all others will be considered to be occupying forces.
He said he'd advised Ukrainian troops to swear allegiance to the Russian army or leave Crimea under safe passage.
In the regional capital, Simferopol, residents have demonstrated this week against the interim government in Kiev, with crowds chanting in favor of Putin.
U.S. visa ban imposed
Amid the rapidly shifting diplomatic sands, European Union leaders are discussing possible economic and diplomatic sanctions against Russia, which could include asset freezes or visa moves.
U.S. Secretary of State John Kerry and European foreign ministers are also meeting in Rome.
The U.S. State Department on Thursday imposed a visa ban on Russian and Ukrainian officials and individuals responsible for, or complicit in, threatening the sovereignty and territorial integrity of Ukraine.
President Barack Obama also signed an executive order laying the groundwork to impose sanctions against individuals and entities responsible for the crisis.
The United States also is adding six F-15 fighter planes to the four currently on a NATO mission in the Baltics, the Pentagon announced Wednesday, and the guided-missile destroyer USS Truxtun is about to take part in a previously scheduled exercise in the Black Sea.
The impact of EU sanctions, if they were imposed, might be felt by other countries, too. Russian lawmakers are drafting a law that would allow the nation to confiscate assets belonging to U.S. and European companies if sanctions are slapped on Moscow, Russian state media reported Wednesday.
The Russian threat was not specific, but numerous large European and U.S. companies have interests in the region, and Russia is a major supplier of natural gas to Europe.
Martin Schulz, president of the EU Parliament, earlier promised in remarks alongside Yatsenyuk that Europe stood behind the new government in Kiev and a peaceful, democratic future for Ukraine.
"We are behind you and your government, and we support you with all our means," he said.
This includes ensuring that an 11 billion-euro aid package offered Wednesday by the European Union gets to Ukraine as soon as possible to shore up the cash-strapped economy, Schulz said.
Tensions remain high around military bases in Crimea, and there are concerns that violence may erupt as tempers fray.
Ukraine's Ministry of Defense said unidentified Russian forces had scuttled an old warship to block seven Ukrainian vessels in a Crimean harbor under cover of darkness Wednesday.
Meanwhile, riot police are in a standoff against pro-Russian demonstrators outside government buildings in Odessa, a port city in southern Ukraine.
And in the eastern city of Donetsk, protesters took over a local government building Wednesday as they called for a referendum on the region's status and greater autonomy, witnesses told CNN.
Russian Foreign Minister Sergey Lavrov earlier criticized the actions of NATO and a regional security bloc, the Organization for Security and Co-operation in Europe
Their actions "are not helping to create an atmosphere of dialogue and constructive cooperation" on Ukraine, he said.
The OSCE said Wednesday it had sent 35 unarmed military observers to Odessa in response to a request from Kiev.
Meanwhile, NATO warned it was reviewing its relationship with Russia and suspending some joint undertakings.
CNN's Michael Holmes reported from Kiev and Laura Smith-Spark wrote and reported in London. CNN's Anna Coren in Simferopol, Matthew Chance in Odessa and Tim Schwarz in Kiev contributed to this report. CNN's Elise Labott, Michelle Kosinski, Susannah Palk, Yon Pomrenze and Catherine E. Shoichet also contributed.
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