The Ukranian defense ministry said pro-Russian "militants" entered a Ukrainian air force base in Crimea, telling the news agency AFP that the armed group were in negotiations with the base's commander.

A Ukranian military official, Vlad Seleznyov, told Reuters that men took over the base - where some 100 Ukrainian troops are stationed - without any shooting, and that no one had been hurt.

All of Ukraine's military bases in the Crimea that have not yet surrendered to the pro-Russian side are understood to be under siege.

Initial reports said the armed men had smashed through the gates of the base, but a Reuters journalist on the scene could not report any signs of the gates being damaged.

OSCE stopped again from entering Crimea

Earlier on Friday, a team of 47 military and civilian observers from the Organization for Security and Co-operation in Europe (OSCE) were stopped from entering Crimea at a checkpoint by armed men. One observer told the AFP news agency that the group was acting under an OSCE mandate "as guests of the Ukranian government."

In a tweet, the OSCE said its "military assessment visitors from OSCE States denied entry into Crimea on Friday, heading back to Kherson to plan next steps."

The Russian foreign ministry said the group was stopped because it had not secured "official invitations" from Crimean authorities.

The OSCE observers had already been denied entry on Thursday, one day after UN envoy Robert Serry was forced to leave Crimea's regional capital Simferopol after being threatened by pro-Russian activists.

Russian parliament vows support for Crimean referendum

Local lawmakers in Crimea this week decided to stage a referendum in March on whether to switch over to Kremlin rule.

That move has been greeted with support by both houses of the Russian parliament, which stated on Friday they would welcome Crimea into the Russian Federation.

But it's prompted dismay from Western allies and Ukraine, who already blame Russia for heightening tensions in Ukraine with the deployment of troops to Crimea last week.

Ban calls for calm

UN Secretary General Ban Ki-moon has labeled the forthcoming referendum a "worrying and serious development" but urged the authorities in Ukraine, "including in Crimea, to treat this matter with calm."

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

The White House, meanwhile, has commented further on a hour-long phone call on Thursday between US President Barack Obama and Russia's President Vladimir Putin. It said Obama told Putin that Russia's actions were in violation of Ukranian sovereignty, leading the US to respond with "several steps ... in coordination with our European partners."

The Kremlin says that during the call, Putin insisted that Moscow's policies with regards to Crimea and eastern Ukraine were fully legal.

"Russia cannot ignore calls for help on this matter and is responding accordingly, in full compliance with international law," the statement said Putin told Obama.

Hollande meets Klitschko

French President Francois Hollande said on Friday no referendum should take place regarding Crimea unless Ukraine decided to hold one.

Hollande met in Paris with Ukraine's former foreign minister Petro Poroshenko as well as ex-boxer Vitali Klitschko, a leading figure of the protest movement that ousted former Ukranian president Viktor Yanukovych.

"The territorial integrity and sovereignty of Ukraine are non-negotiable," Hollande said, adding that the West's response to the crisis would be "modulated according to the situation."

Gabriel travels to Kyiv

Germany's Vice-Chancellor, Sigmar Gabriel, traveled to Independence Square in Kyiv on Friday, and called on Europeans to support Ukraine.

The interim government in Kyiv says it needs at least 25 billion euros ($34.6 billion) over the next two years to avoid bankruptcy.

Gabriel had been in Moscow a day earlier, becoming the first high-ranking Western official to hold talks with Putin since the Crimea crisis began. He said the Russian leader was noncommittal about establishing a contact group - consisting of diplomats from major world powers - to push for negotiations between Russia and Ukraine.

Gazprom threatens to turn off gas

Russian export gas giant Gazprom warned Ukraine on Friday that it needed to pay a bill for its gas debt, otherwise the supply could be cut off.

"Ukraine has de-facto stopped paying for gas ... We cannot deliver gas for free. Either Ukraine pays the debt and pays for current supplies or the risk appears of a return to the situation at the start of 2009," Russian news agencies quoted Gazprom's chief executive, Alexei Miller, as saying.

"Gazprom has not received the payment on its account. The debt due has increased and is now $1.89 billion (1.36 billion euros)," he added.

The threat is an escalation of a warning Gazprom made earlier in the week, that from April it would cancel a discount it gives Ukraine on the price of gas.

jr/ph (Reuters, AFP, AP, dpa)