BELBEK, Ukraine (AFP) - Russian forces fired warning shots on Ukrainian soldiers at a base near Sevastopol on Tuesday as tensions flared in the standoff over Crimea.

It appeared to be the first shots fired since Russian forces began surrounding Ukrainian military facilities in the flashpoint peninsula.

Pro-Russian troops guarding an airfield in Belbek, near Sevastopol, fired the shots in the air as a column of about 300 Ukrainian troops approached around 9am local time (0700 GMT), Ukrainian officers said.

The column, waving a Ukrainian flag and in uniform but unarmed, tried to walk up a hill to the airfield, said one of the officers involved who gave his name only as Alexander.

"We came across some unidentified soldiers who told us: 'Stop! Don't come any further'," Alexander said.

"They shot in the air. We are on our territory, we are defending our country, so we moved forward 50 metres," he said.

"Then they shouted: 'Stop! We have orders to shoot in the legs.' So we stopped and our commander went to negotiate." After talks with the soldiers, the Ukrainian servicemen returned to their base - which houses the Sevastopol tactical aviation brigade - where they were welcomed by applause from gathered loved ones, he said.

Later inside the base, Ukrainian soldiers said they had no intention of picking a fight with the Russian forces.

"We will have to be patient, we will stay here and wait. Die if we have to," Alexander said.

There seemed little chance they would be able to hold off an assault if one came.

Makeshift barricades had been erected with overturned desks and sandbags, but no defensive positions were being manned and the soldiers were wandering amidst administrative buildings and dormitories, their hands in their pockets.

Russian forces have surrounded Ukrainian military bases across Crimea as the Russian-speaking autonomous region has been thrown into turmoil following the ouster last month of Moscow-backed president Viktor Yanukovych.

Tensions are high, but eased slightly on Tuesday morning after fears of an imminent assault by Russian forces did not materialise overnight.

Ukrainian officials said Monday that Russia had given Ukrainian soldiers in Crimea an ultimatum to surrender or face an all-out assault, although Russia denounced the claim as "complete nonsense".

In Sevastopol, a bastion of pro-Kremlin sentiment, pro-Russian activists had surrounded the headquarters of the Ukrainian navy in Crimea.

An AFP reporter said about 100 pro-Russia activists were gathered outside the offices and formed a human chain, as Russian forces with automatic rifles stood nearby.

Ukrainian defence officials said Russian vessels were also blocking Ukrainian warships from trying to leave the port, which has been the home of the Russian navy's Black Sea fleet for some 250 years.

Ukraine says several thousand Russian troops have been sent by the Kremlin to Crimea, but Russian President Vladimir Putin denied that on Tuesday, saying that only "local forces of self-defence" were surrounding Ukrainian military bases.

Armed men believed to be under Moscow's orders last week seized key government buildings in Crimea and airports on the peninsula.

Regional lawmakers deposed the Kiev-supporting prime minister and have called for a referendum on March 30 on proclaiming greater autonomy for Crimea.