Pro-Russian leaders in Crimea made final preparations on Saturday for a referendum widely expected to transfer control of the Black Sea region from Ukraine to Moscow, despite an outcry and threat of sanctions from the West.

Russia vetoed a UN Security Council resolution that declared the referendum invalid, as Ukraine's defense ministry scrambled aircraft and paratroopers to confront what it said was a Russian encroachment just beyond Crimea's formal regional boundary.

Ukraine's new rulers accused "Kremlin agents" of fomenting violence in the Russian-speaking east of the country. They urged people not to respond to provocations that Kiev fears Moscow may use to justify further incursions after its takeover of Crimea.

Russia issued a new statement saying it was ready to protect Ukrainians from nationalist militants who it said were threatening eastern cities.

Sunday's vote in Crimea, dismissed as illegal by Kiev and Western governments, has triggered the worst East-West crisis since the Cold War. It also marks a new peak in turmoil in Ukraine that goes back to November, when the now-ousted president, Viktor Yanukovich, walked away from a trade deal with the European Union.

"This annexation ... goes beyond Ukraine. It concerns us all," France's UN ambassador said after the Security Council vote. U.S. President Barack Obama is sending Vice President Joe Biden with a message of reassurance to Poland, a former Soviet bloc state, and the Baltic states, which until 1991 were ruled by Moscow.

Putin's spokesman Dmitry Peskov insisted there should be no new Cold War in an age of globalization and economic interdependence: "We hope with all our heart that both we and our partners have enough political wisdom and sense of political realism to avoid sliding into an even deeper confrontation."

A U.S. guided-missile destroyer is extending exercises in the Black Sea. Along with NATO air patrols on Ukraine's western border, such maneuvers aim to send signals of resolve.

Eastern tensions

Though the situation was calm in Crimea ahead of the vote, tensions remained high in eastern Ukraine, where two people were killed in Kharkiv late on Friday.

Crimean Prime Minister Sergei Aksyonov, whose election in a closed session of the regional parliament is not recognized by Kiev, said there were enough security personnel to ensure the poll would be safe.

"I think we have enough people - more than 10,000 in the self-defense forces, more than 5,000 in different units of the Interior Ministry and the security services of the Crimean Republic," he told reporters.

In Kiev, the Ukrainian parliament voted to dissolve Crimea's regional assembly, which has organized the referendum and backs union with Russia. But the move was symbolic, as Kiev has lost both political and military control of the peninsula.

On Kiev's Independence Square or Maidan - heart of the revolt against the Moscow-backed Yanukovich - hundreds of people chanted "Crimea is Ukraine! Crimeans, we support you!"

Aksyonov and Moscow do not officially acknowledge that Russian troops have taken control of Crimea. They say that thousands of unidentified armed men, visible across the region, belong to "self-defense" groups created to ensure stability.

But the Russian military has done little to hide the arrival of thousands of soldiers, along with trucks, armored vehicles and artillery. Masked gunmen surrounding Ukrainian military bases in Crimea have identified themselves as Russian troops.

Ukraine's military said it confronted Russian forces which crossed Crimea's regional border on a remote sand spit, some 30 km (20 miles) off the mainland. Crimea's separatist government said these were its own 'self-defense forces', which had moved to defend a gas pumping station.

Ukrainian officials said no shots were exchanged.

"Real danger" of invasion

Moscow leases the Crimean port of Sevastopol from Kiev to station its Black Sea Fleet. Under the deal it can station up to 25,000 troops there but their movements are restricted onshore.

Polling stations will open at 8 a.m. (0600 GMT) on Sunday and close 12 hours later. Preliminary results are expected within hours of polls closing on Sunday night.

Crimea's electorate of 1.5 million will choose between two options - but both imply Russian control of the peninsula.

Many of the ethnic Russians who form the majority on the peninsula seem likely to back the first choice on the ballot, union with Russia, if only for economic reasons. A second option is independence, initially within Ukraine, that Crimea's new leaders say they will use as a basis for accepting Russian rule.

Ethnic Tatars - Sunni Muslims of Turkic origin who make up 12 percent of Crimea's population - have said they will boycott the referendum, despite promises by the authorities to give them financial aid and proper land rights.

Acting Ukrainian president Oleksander Turchinov referred to three deaths in two days in two eastern cities - Donetsk and Kharkiv - and said there was "a real danger" of invasion by Russian troops across Ukraine's eastern border.

Both cities have large numbers of Russian-speakers - significant since Russian President Vladimir Putin has vowed to protect ethnic Russians and Russian-speakers in Ukraine. His foreign ministry said on Saturday it would consider requests from people in Ukraine for protection - similar language to that used to justify Moscow's move in Crimea.

Addressing members of Yanukovich's party, Turchinov said: "You know as well as we do who is organizing mass protests in eastern Ukraine - it is Kremlin agents who are organizing and funding them, who are causing people to be murdered."

Two men, described by police as pro-Russian demonstrators, were shot dead in a fight in Kharkiv late on Friday. A Ukrainian nationalist was stabbed to death when pro-Russia and pro-Ukraine demonstrators clashed in Donetsk on Thursday.

Turchinov told parliament: "There is a real danger from threats of invasion of Ukrainian territory."

Defending "compatriots"

Russia has refused to recognise the new Western-backed interim administration in Kiev. Its foreign ministry unnerved the new authorities in Kiev by saying clashes in Donetsk showed Ukrainians had lost control of the situation there.

Ukraine's interior minister accused ousted president Yanukovich of promoting unrest with "extremist Russian forces". Arsen Avakov issued an appeal on Facebook: "Don't let them manipulate you! ... This isn't a game of toy soldiers."

Two men, aged 21 and 30, were killed in Kharkiv by buckshot late on Friday when pro-Russian demonstrators besieged an office of the far-right Ukrainian nationalist group Right Sector, which rose to prominence fighting riot police in Kiev over the winter.

Police said 32 Right Sector activists and six pro-Russian demonstrators were detained and a number of weapons seized.

Kharkiv governor Ihor Baluta, newly appointed by the interim authorities in Kiev, called the incident a "well-planned provocation by pro-Russian activists."

The Russian Foreign Ministry said it had information that armed Right Sector militants were opening an "eastern front" and planning to reinforce their activists in eastern cities.

Western powers, preparing economic sanctions against Russia over Crimea, largely dismiss Russia's characterization of the new authorities in Kiev as the successors of Nazi-allied Ukrainian forces which fought the Red Army in World War Two.

Authorities in Kharkiv banned political gatherings that were planned in the city over the weekend. In Donetsk, hundreds of people rallied in Lenin Square, flying Russian flags and calling for a referendum in the region similar to that in Crimea.

In Moscow, a large crowd estimated by witnesses at some 30,000 demonstrated against Russia's action in Ukraine, in the biggest protest against Putin in two years - a sign his moves against Ukrainian leaders brought to power by street protests might be a rallying point for the jaded Russian opposition.