Ukraine crisis: US warns Russia not to annex Crimea
The US has warned Russia that any moves to annex Crimea would close the door to diplomacy.
US Secretary of State John Kerry told Russian counterpart Sergei Lavrov that Crimea is part of Ukraine and Moscow should avoid military escalation.
Meanwhile, US President Barack Obama has been discussing the deepening crisis with world leaders.
It comes as warning shots were fired as a team of international observers was turned back from entering Crimea.
The Organisation for Security and Co-operation in Europe (OSCE) said that no-one was hurt in the incident at Armyansk.
It was the third time the OSCE has been prevented from entering Crimea, now in the control of pro-Russian forces.
Moscow has been tightening its military grip on the Crimean peninsula, and the pro-Russian authorities there have called a 16 March referendum to secede from Ukraine and join Russia.
The exchange between Mr Kerry and Mr Lavrov came in a telephone conversation on Saturday, a US State Department official said.
"He (John Kerry) made clear that continued military escalation and provocation in Crimea or elsewhere in Ukraine, along with steps to annex Crimea to Russia would close any available space for diplomacy, and he urged utmost restraint," the official said.
Russian President Vladimir Putin has insisted he has the right to protect Russian interests and the rights of ethnic Russians in Crimea.
Mr Lavrov had earlier said that the crisis in Ukraine was "created artificially for purely geopolitical reasons".
He confirmed that Russia had contacts with Ukraine's interim government but said Kiev was beholden to the radical right.
Speaking to reporters in Moscow on Saturday, he said: "We are ready to continue a dialogue [with the West] on the understanding that a dialogue should be honest and partner-like, and without attempts to make us look like a party to the conflict. We didn't create this crisis."
As diplomatic efforts to resolve the crisis continued, President Obama held individual conversations on Saturday with British Prime Minister David Cameron, Italian Prime Minister Matteo Renzi and French President Francois Hollande, the White House said.
"The leaders reiterated their grave concern over Russia's clear violation of international law and reaffirmed their support for Ukraine's sovereignty and territorial integrity," a statement said.
Mr Obama also held a conference call with Latvian President Andris Berzins, Lithuanian President Dalia Grybauskaite and Estonian President Toomas Hendrik Ilves. The three Baltic states were all part of the former Soviet Union.
Earlier, the French presidency said Mr Hollande and Mr Obama discussed "new measures" against Russia if it failed to act to defuse the crisis.
They insisted on the "need for Russia to withdraw forces sent to Crimea since the end of February and to do everything to allow the deployment of international observers," Mr Hollande's office said.
It was not clear what "new measures" could entail.
Witnesses travelling with the OSCE said several shots were fired in the air as a convoy of vehicles approached a checkpoint manned by pro-Russian forces on a road leading from mainland Ukraine into the Crimea peninsula.
An OSCE spokeswoman said that the mission was withdrawing to the nearest big city, Kherson, to decide on its next steps.
The Vienna-based OSCE was invited by Ukraine's interim government, but Russian separatist authorities in Crimea say it does not have permission to enter the region.
Volunteers sworn in
Ukraine's crisis began in late November when President Viktor Yanukovych abandoned a landmark deal with the EU in favour of closer ties with Russia.
After weeks of protests in which more than 100 protesters were killed, Mr Yanukovych fled Ukraine and opposition leaders formed an interim government.
Russia effectively took control of Crimea - where its Black Sea Fleet is based - more than a week ago.
In the Crimean capital Simferopol on Saturday, the man named as prime minister of Crimea, Sergei Aksyonov, held a ceremony to swear in pro-Russian self-defence volunteers as members of what he called a new Crimean army.
Witnesses said about 30 armed men, from a wide age range, were sworn in at a city park.
Reports suggested Russia was ramping up its military presence in Crimea on Saturday.
Dozens of military trucks transporting heavily armed soldiers arrived at a military airfield at Gvardeiskoe north of Simferopol on Saturday, AP news agency reported.
Licence plates and numbers indicated they were from the Moscow region, the report said.
In a more positive development on Saturday, Russia's deputy foreign minister held talks with Ukraine's ambassador in Moscow.
The foreign ministry gave no details but said the talks on Saturday between deputy minister Grigory Karasin and ambassador Volodymyr Yelchenko were held in an "open atmosphere".