Militants Smash into Ukrainian Base as Armed Men Block OSCE Members' Entry to Crimea

Pro-Russian militants smashed through the gates of a Ukrainian air force base in Crimea on Friday in the latest signs of boiling tensions on the rugged peninsula, Ukraine's defense ministry said.

The militants entered the base and were in negotiations with the commander of the base, while soldiers guarding it have locked themselves in their barracks, a ministry spokesperson told Agence France Presse.

No shots were fired in the incident. Another Ukrainian military official said the incident began at 1740 GMT.

The base is a command center for a tactical group of Ukraine's air force.

Earlier on Friday, armed men at a checkpoint flying the Russian flag blocked OSCE observers from entering the peninsula for a second day on Friday, an AFP reporter said.

The team of 47 military and civilian observers from the Organization for Security and Cooperation in Europe arrived at the checkpoint near the village of Chongar around 3:00 pm local time (1300 GMT) but were prevented from advancing.

They were hoping to hold talks with the unidentified armed men to allow their convoy to pass, one of the observers said.

"We are just trying to go through here as guests of the Ukrainian government under an OSCE mandate," the observer, who spoke on condition of anonymity, told AFP.

"We're going to try and negotiate with these people here."

The two buses stopped dead at a checkpoint flying the Russian colors after being trailed by a convoy of cars waving the Ukrainian flag and honking their horns.

Led by a Ukrainian police car, the convoy of two buses and about 100 cars approached the improvised roadblock near the village of Chongar around 3:00 pm local time (1300 GMT).

Forced to stop by concrete slabs interspersed on the road, the observers waited as attempts were made to convince the armed men to allow the convoy through.

Out of the cars spilled dozens of people, waving flags and chanting "Crimea is Ukraine!"

"We don't want Ukraine to be split, the OSCE is able to negotiate, we want a decision to be made peacefully," said one of the demonstrators, 55-year-old Lyudmila Korbec.

As the crowd gathered between the buses and the checkpoint, tensions started to mount.

Eventually one of the demonstrators shouted out above the crowd, urging them to leave and warning of a possible "provocation".

The crowd thinned as the demonstrators drove back the way they came. A source within the OSCE mission said talks were continuing.

But it soon became clear that there was no way the armed men, their faces hidden behind balaclavas, were going to allow the convoy through.

The buses turned around and returned to the city of Kherson, where the team had spent the night after being blocked from entering Crimea a first time on Thursday.

"They didn't allow us to enter and recommended we talk to the government of Crimea. They said the Crimean government didn't invite the mission," said a Ukrainian military officer travelling with the convoy.

"Discussions are to be held on whether we will make another attempt."

The unarmed observers, from 25 of the organization's 57 members, have been invited by Ukraine's new government for a mission aimed at monitoring the situation and lowering tensions in Crimea.

However, Russia explained that the OSCE observers had failed to obtain "official invitations" from the Crimean authorities.

The OSCE tried to enter Crimea "bypassing the principle of consensus that is fundamental to the OSCE, without considering the opinions and recommendations of the Russian side, without waiting for official invitations from the Crimean side," the Russian foreign ministry said.

On Thursday, gunmen had also prevented the observers from 21 of the organization's 57 member countries from crossing into the peninsula.

The Russian ministry accused the OSCE of acting "in the worst traditions of double standards on the situation in Ukraine" and turning a blind eye to "violent acts by extremist forces."

Moscow called for the OSCE to condemn "violent acts" by the new authorities in Kiev and to declare their takeover of power as illegal.

The OSCE's unarmed observer mission is aimed at defusing tensions in Crimea, which was taken over by Russian forces over the weekend.

Russian forces have surrounded Ukrainian military bases on the peninsula and local lawmakers have decided to renounce ties with Ukraine and stage a March 16 referendum on switching over to Kremlin rule.

Also on Friday, pro-Russian militants put up a barricade of crates outside the Ukrainian navy headquarters in Sevastopol in southern Crimea, an AFP reporter on the scene saw.

Ukrainian soldiers "can come out if they want but if they do so we will not let them back in," said one militant, Dmitry, who identified himself as a member of a pro-Moscow "self-defense group".

The barricade had a Russian flag planted on the top and the militants wore red armbands emblazoned with the words "Russian Block" written on them.

Inside the base, unarmed Ukrainian soldiers could be seen behind the gates pacing with their hands in their pockets.

In the port of Sevastopol, the base of Russia's Black Sea Fleet for 250 years, a Russian warship could be seen moored near two Ukrainian navy ships which have also been blockaded.

Jutting into the Black Sea, Crimea is easy to isolate from the rest of Ukraine.

Only two main roads lead into the peninsula, one crossing the narrow Isthmus of Perekop in the west and another eastern route on the Azov Sea.

At the checkpoint near Chongar, which is on the eastern route, it appeared that preparations were being made for a more permanent border to be put in place.

Spreading out from the road by the checkpoint was a perfect row of holes dug into the earth, each about half-a-meter wide, in what looked like the first steps of installing a border fence.

Signs nearby warned of mines and were marked "Stop! Danger to Life!" with a skull-and-crossbones.

Meanwhile, U.N. Secretary General Ban Ki-moon described the Crimean parliament's decision to call a referendum on whether to leave Ukraine and join Russia, a "worrying and serious development."

Ban's spokesman said Friday that the U.N. chief was urging the authorities in Ukraine, "including in Crimea, to treat this matter with calm," referring to the March 16 vote to renounce ties with Kiev and switch to Kremlin rule.

Ban "believes all concerned should think about the implications of any hasty action or decision taken in the heat of the moment," U.N. spokesman Martin Nesirky said. "He cannot underline enough the need for peace and stability in the region."

Nesirky, however, did not state if the United Nations would consider such a referendum to be illegal under international law or the Ukrainian constitution, as has been said by the United States and its allies.

The heads of Russia's two houses of parliament said they would respect a decision by lawmakers in the flashpoint Black Sea region to renounce ties with Kiev and stage the referendum.

The escalating threat of Ukraine being partitioned between its pro-European west and more Russified southeast prompted US President Barack Obama to place an hour-long call to Russian counterpart Vladimir Putin.

The White House said Obama "emphasized that Russia's actions are in violation of Ukraine's sovereignty and territorial integrity, which has led us to take several steps in response, in coordination with our European partners."

The European Union has strengthened its resolve to impose stiff sanctions on Russia while also vowing to sign an historic trade pact aimed at pulling Kiev out of Moscow's orbit before Ukraine holds snap presidential polls on May 25.

But with Russian forces in effective control of Crimea -- a predominantly ethnic Russian peninsula roughly the size of Belgium and the base of the Kremlin's Black Sea Fleet -- the threat of Ukraine's division seems more real than at any point since Putin won parliamentary approval to use force against his western neighbor.