A pro-Russian force opened fire in seizing a Ukrainian military base in Crimea on Monday and NATO announced reconnaissance flights along its eastern frontiers as confrontation around the Black Sea peninsula showed no sign of easing.
Ukrainian activists trying to cross into Crimea to show solidarity with opponents of last week's Russian military takeover there said they were halted by men in uniforms of the now outlawed riot police.
One of these fired at close range, hitting a man in the chest, apparently with rubber bullets.
With diplomacy at a standstill, Russia said the United States had spurned an invitation to hold new talks on resolving the crisis, the worst East-West standoff since the Cold War - although Washington said later a meeting of foreign ministers was possible this week, if Moscow shows it is ready to "engage".
The U.S.-led NATO defence alliance said AWACS early warning aircraft, once designed to counter feared Soviet nuclear missile strikes, would start reconnaissance flights on Tuesday over Poland and Romania to monitor the situation in Ukraine, flying from bases in Germany and Britain.
The United States on Tuesday will also begin previously planned military training exercises in the region, the first since the Russian intervention in Crimea.
A U.S. Navy destroyer will participate in manoeuvres with Romanian and Bulgarian warships in the Black Sea, across from Crimea. In Poland, U.S. fighter jets will take part in joint exercises.
British Prime Minister David Cameron told Germany's Bild newspaper, however, that Western powers were not considering military action and wanted a diplomatic solution. European Union governments are considering sanctions against Russia.
Ukrainian Prime Minister Arseny Yatseniuk, who said he would address the U.N. Security Council on Thursday, blamed the crisis on Russia and accused Moscow of undermining the global security system by taking control of Crimea.
Ukraine's new justice authorities issued warrants for the arrest of Crimea's pro-Russia leaders on Monday, six days before a referendum they have called to join the region to Russia.
Russian forces have in little more than a week taken over military installations across Crimea, home to the Russian Black Sea Fleet and Russian territory until Soviet leader Nikita Khrushchev gave it to Ukraine in 1954.
Pro-Russian separatists have taken control of the regional parliament, declared Crimea part of the Russian Federation and announced a referendum for Sunday to confirm that.
President Vladimir Putin says Moscow is acting to protect the rights of ethnic Russians, who make up a majority of Crimea's population, after Ukraine's president, Viktor Yanukovich, was ousted last month in what Russia calls a coup.
On Monday, a Ukrainian defence official said a Russian-led military force of about a dozen men fired in the air as they took control of a Ukrainian naval base near the town of Bakhchisaray, though no one was hurt.
The force was accompanied by the base's Ukrainian commander. He persuaded a number of his men to join the Russian forces while allowing others who refused to leave, the Ukrainian official, Vladislav Seleznyov wrote on Facebook. The Russian force later drove off with nine Ukrainian vehicles.
Yarik Alexandrov, one of the Ukrainian naval personnel who refused to pledge allegiance to Moscow, told Reuters near the base that he and his comrades at first refused to surrender.
"Then they started shooting round our feet and we surrendered," he said. "What could we do? We had no weapons."
Similar small confrontations have taken place at other Ukrainian bases around Crimea, although shooting has been rare and there has so far been no bloodshed. Russia denies its troops are involved - a stance ridiculed in Kiev and the West.
In a sign of the peninsula's growing isolation from the Ukrainian mainland, armed men prevented a convoy of cars from a Ukrainian activist group crossing into Crimea.
The group was part of the Maidan movement behind the protests that forced Yanukovich to flee to Russia. Ukrainian television showed men in the uniform of the Berkut riot police, banned by the new authorities for its role in shooting dozens of demonstrators in Kiev last month, blocking the road south.
One was shown firing twice, hitting a man in the chest. His injuries appeared minor, suggesting the use of rubber bullets.
In other armed action, Russian forces took over a military hospital and a missile unit. Reuters correspondents also saw a big Russian convoy on the move just outside the port city of Sevastopol near a Ukrainian air defence base.
It comprised more than 100 vehicles, including around 20 armoured personnel carriers, plus mobile artillery.
CHANCE OF TALKS SPURNED
Putin says Russia is not controlling events in Crimea, but denials of Russian involvement are rejected by the United States as the two former Cold War enemies wage a geopolitical battle over the future of Crimea and Ukraine.
Russian Foreign Minister Sergei Lavrov told Putin that Russia's position on Ukraine remained at odds with the West, but U.S. Secretary of State John Kerry had declined an invitation to visit Russia on Monday for further talks.
"It is all being formulated as if there was a conflict between Russia and Ukraine ... and our partners suggested using the situation created by a coup as a starting point," Lavrov told Putin during talks in the Black Sea resort of Sochi.
He did not say why Kerry had postponed the talks.
The State Department said Kerry told Lavrov on Saturday that Washington wanted Moscow to cease its drive to annex Crimea and end "provocative steps".
In a statement, it added: "Kerry made clear to Foreign Minister Lavrov that he would welcome further discussions focused on how to de-escalate the crisis in Ukraine if and when we see concrete evidence that Russia is prepared to engage on these proposals."
In Kiev, Yatseniuk said he would address the U.N. Security Council during a debate on Ukraine. He is also due to hold talks with the U.S. government that will show Washington's support of the new Ukrainian leadership.
"Russia's policy is aimed at undermining the basis of the global security system and revising the outcome of World War Two," Interfax quoted Yatseniuk as telling reporters.
Western powers have rallied behind Ukraine's new leaders and the World Bank said on Monday it planned to provide up to $3 billion this year to see Kiev through an economic crisis.
U.S. senators are preparing legislation that aides said would be broader than a measure passed last week by the House of Representatives backing $1 billion in loan guarantees for Ukraine, and could include sanctions.
Ukraine's crisis was triggered in November by Yanukovich's refusal, under Russian pressure, to sign deals on closer political and trade ties with the European Union.
Although three months of protests against Yanukovich were mostly peaceful, at least 80 demonstrators were killed in clashes after police used force against them, some by sniper fire.
Yanukovich fled Ukraine before a peace deal with the opposition was implemented, and a new national unity government was installed. He is wanted for mass murder in Ukraine and is being sheltered by Russia.
WEST DOES NOT RECOGNISE REFERENDUM
Western countries have denounced the Russian intervention in Crimea and say the borders of Ukraine, a country of 46 million, should remain unchanged. They have said they will not accept the outcome of Sunday's vote.
"The United States is not prepared to recognise any result of the so-called referendum taking place in six days' time," U.S. Ambassador Geoffrey Pyatt said in Kiev.
"We are committed to Crimea's status as part of Ukraine. The crisis needs to be solved diplomatically, not militarily."
In the latest military movements, in Sevastopol, where Russia has its Black Sea Fleet base, Russian forces disarmed servicemen at a Ukrainian army missile base, Seleznyov said.
He told Fifth Channel television that about 200 soldiers aboard 14 trucks moved on the building at about 1.30 a.m and threatened to storm it if the Ukrainian soldiers failed to give up their weapons.
In the eastern city of Luhansk, Ukraine's security services said they were investigating the takeover on Sunday of the main administrative building.
The region's top official was held captive in a room where he was made to write a letter saying he had resigned, but he later said he was still performing his duties.