Russian forces that have surrounded the military bases in Ukraine’s autonomous region of Crimea have given ultimatum to Ukrainian navy forces there to surrender, according to the headquarters of the Ukrainian navy in Sevastopol.
The claim, which requested the forces to surrender by 0300:GMT on Tuesday or face a military assault, has been denied by Russian sources.
A commander at the headquarters confirmed to Al Jazeera the ultimatum from Russian forces to lay down their weapons, adding that the order includes two main bases in the region and ships patrolling the bays of Kurinaya and Streletskaya.
The headquarters did not confirm if the ultimatum only includes the navy or the whole Ukrainian military forces in Crimea.
Al Jazeera correspondents in Kiev and Crimea say that the ultimatum is likely to include the army.
Al Jazeera's Hoda Abdel-Hamid, reporting from Crimea, said she did not get the impression that the armed forces were considering surrendering.
"A commander that talked to us said they only received orders from Kiev," she said.
At an emergency meeting of the UN Security Council on Monday, Vitaly Churkin, Russia's envoy to the body, said that Viktor Yanukovych, Ukraine's ousted president, had requested Russia's armed forces to enter Ukraine and establish law and order country.
Churkin quoted Yanukovych as saying "the life and security and the rights of people, particularly in the southeast part in Crimea, are being threatened" and that there were "open acts of terror and violence."
In response, Samantha Power, the US ambassador, told the meeting: "One might think that Moscow has just become the rapid response arm of the High Commissioner for Human Rights."
A letter from Ukraine's UN mission circulated during the emergency meeting said that 16,000 Russian troops have now been deployed in the Crimea region and called for international help.
Earlier, Russia's Interfax news agency initially also reported that Russia's Black Sea Fleet in Crimea had told Ukrainian forces in Crimea to surrender.
The ultimatum, Interfax said, was issued by Alexander Vitko, the fleet's commander, quoting a Ukrainian Defence Ministry as the source.
Later, Interfax quoted a Russian official at the fleet's headquarters as saying that they have not issued an ultimatum.
"This is complete nonsense," the military official was quoted as saying.
The West is facing its biggest confrontation with Moscow since the Cold War after Russian President Vladimir Putin declared the right to use force in its neighbour and Ukraine put its military on alert.
Ukrainian border guards have said that Russia has started a build-up of armoured vehicles on the Russian side of a narrow stretch of water between Russia and Crimea.
According to Kiev’s statements, Russian troops continued to pour into Crimea on Monday and took positions outside military installations in the peninsula.
Last week, unidentified gunmen, believed to be Russians, gained control of state buildings and airports, in Crimea, a Black Sea peninsula where ethnic Russians are the majority and Moscow has a naval base.
Moscow defends military action
Sergei Lavrov, the Russian foreign minister, has refused international calls for Russian army to pull out to their bases in Crimea.
Speaking at the opening of the UN Human Rights Council in Geneva, Lavrov justified the use of troops in Crimea as a necessary protection for his country's citizens living there, and called on Kiev to abide by a European Union sponsored deal between the Ukrainian opposition and Viktor Yanukovich, the country's deposed president.
Barack Obama, the US president, said that Russia had violated international law in its military intervention in Ukraine, and said Washington will look at a series of economic and diplomatic sanctions that would isolate Moscow.
Putin needs to allow international monitors to mediate a deal in Ukraine acceptable to all Ukrainian people, Obama told reporters before he met with Israeli Prime Minister Benjamin Netanyahu on Monday.