An unarmed observer mission of the Organisation for Security and Co-operation in Europe (OSCE) has been forced to turn back from crossing into Ukraine's Crimea region after gunmen fired warning shots, the European security body said.

Nobody from the mission was hurt in the third straight day that the civilian and military OSCE observers had been turned back from their attempts to cross the narrow isthmus connecting the Black Sea peninsula to the rest of Ukraine.

Sources told Al Jazeera that two sets of warning shots were fired.

An OSCE spokeswoman said in an e-mailed statement that the mission was withdrawing to Kherson, the nearest big city, to decide on its next steps, Reuters news agency reported.

Russian forces seized control of the region last week and President Vladimir Putin declared that Moscow has the right to take military action in Crimea to protect the Russians and Russian-speaking population there.

The OSCE mission has been invited by Ukraine's government, but the pro-Russian Crimean authorities, who voted to join Russia on Thursday, say they have not given it permission to enter the region.

Meanwhile, a large unmarked military convoy made its way towards the Crimean regional capital of Simferopol, Associated Press news agency reported.

At least 60 military vehicles bearing no licence plate numbers were seen on the road from Feodosia, in eastern Crimea, to Simferopol, the agency said, adding that the convoy carried heavily armed soldiers who appeared to have no identifying badges or insignia.

In another development on Saturday, the Russian foreign ministry has announced that Grigory Karasin, Ukraine's envoy to Moscow, has met Volodymyr Yelchenko, Russia's deputy foreign minister, for talks on the situation in Crimea in an "open atmosphere".

Russia says its only troops in Crimea are those normally stationed there with its Black Sea Fleet in line with a bilateral agreement, an assertion Washington calls "Putin's fiction". Kiev says there are 30,000 Russians in Crimea while the United States Department of Defense estimates their number at around 20,000.