European Union leaders have called a special summit for Thursday, where they are expected to freeze visa liberalization and economic cooperation talks with Russia if Moscow hasn't taken steps by then to calm the crisis in Ukraine's Crimean peninsula.
Before that, NATO allies will hold emergency talks on the crisis on Tuesday, the alliance announced. The meeting was called by Poland, which invoked a NATO rule allowing any ally to consult with the others if it feels its security, territorial integrity or independence are under threat. Poland borders on Ukraine.
In Washington, President Barack Obama said his government will look at a series of economic and diplomatic sanctions that would isolate Moscow. Russian President Vladimir Putin needs to allow international monitors to mediate a deal in Ukraine acceptable to all Ukrainian people, Obama told reporters before meeting with Prime Minister Benjamin Netanyahu.
Meanwhile, in Moscow a spokesman for the Russian Defense Ministry dismissed as “total nonsense” Ukrainian statements that Moscow had issued an ultimatum for the surrender of Ukrainian forces in the Crimea.
Interfax-Ukraine reported earlier that Russian Black Sea Fleet Commander Aleksandr Vitko had warned that "a real assault on the units and detachments of the Armed Forces of Ukraine" would begin if the forces did not surrender by 5 A.M on Tuesday.
On Monday night it was reported that armed men had seized a Ukrainian checkpoint at a terminal of the ferry between the Crimea and Russia and that three truckloads of soldiers had crossed to Ukraine from the Russian side.
Ukraine's interim president Oleksander Turchinov said the situation was "difficult" in some regions in the south and east of the country, where there are many Russian speakers, but that the Ukrainian authorities had matters there under control.
He also said Russia's Black Sea Fleet had trapped Ukrainian navy vessels in the bays of Sevastopol, the Crimean port where the Russian fleet has a base.
"The situation in Crimea remains tense and Russia's military presence is growing," Turchinov told a news briefing. "I appeal to Russia's leadership - stop the provocative actions, aggression and piracy. This is a crime and you will answer for it."
Meeting in Brussels, EU foreign ministers said they had also halted preparations for the G8 summit, which is set for June in the Russian resort of Sochi. Foreign policy chief Catherine Ashton said the EU would give Russia until Thursday to show clear signs of goodwill, including a willingness to open talks and a withdrawal of Russian troops to their barracks in the Crimea.
"We need to see a return to barracks by those troops that have currently moved (from) where they have been staying," Ashton told reporters after the foreign ministers' meeting in Brussels. "There are serious concerns about overflights, about reports of troops and armed personnel moving."
The seizure of Crimea has created the greatest confrontation between Russia and the West since the collapse of the Soviet Union in 1991, an event Putin once described as the worst geopolitical catastrophe of the 20th century.
"In the absence of de-escalating steps by Russia, the EU shall decide about consequences for bilateral relations between the EU and Russia, for instance suspending bilateral talks ... on visa matters ... and will consider further targeted measures," the ministers said in a statement.
Russia and the European Union have been discussing visa cooperation since 2007, with Russia keen to have visa-free access to the EU's member states. It is an issue raised at nearly every meeting between Moscow and Brussels.
Moscow, which effectively took control of the Crimea over the weekend, has said it is protecting the lives of Russian citizens and speakers in Ukraine, and appears to be calculating that the West cannot afford to risk a wider conflagration by taking anything approaching military action.
Many Europeans are concerned about pushing Putin too far, mindful of their economic links with Russia, including a heavy dependence on Moscow's gas and oil exports. There is also concern about the time required for sanctions and the legal hurdles that must be cleared.
Russia is the EU's biggest trading partner after the United States and China, with 123 billion euros ($170 billion) of goods exported there in 2012. It is also the EU's most important single energy supplier, accounting for more than a quarter of all EU consumption of oil and gas.
Speaking in Moscow earlier Monday, the speaker of Russia's lower house of parliament said that "for now, there is no need" to send the Russian armed forces into Ukraine.
On Saturday, President Vladimir Putin secured the parliament's permission to use the military in Ukraine if he wishes, citing the need to protect Russians in the neighbouring nation.
"The decision ... only gives the right (to use the armed forces), which can be exercised in case of necessity; for now there is no need," State Duma speaker Sergei Naryshkin told the state-run Rossiya-24 television in an interview.
Elsewhere in Ukraine, pro-Russian demonstrators occupied the regional government building in the eastern city of Donetsk on Monday, as besieged lawmakers voted to hold a referendum, without saying what the public would be asked to decide upon.
The chaotic scene in the heart of Ukraine's Russian-speaking east was one of the boldest actions yet by pro-Russian youths after several days of rallies in eastern and southern cities that Kiev says are organized by Moscow as a pretext to invade.
Talk of Invasion
Russian forces have already taken control of Ukraine's Crimea region, an isolated Black Sea peninsula, and Moscow has threatened to invade Ukraine to protect Russian speakers from what it says is a nationalist new government in Kiev.
Kiev says pro-Russian demonstrations have been organized by Moscow as a pretext to invade. Donestsk is one of the most industrialized parts of Ukraine, producing coal, steel, chemicals and turbines for nuclear plants.
It is also the home city of Viktor Yanukovych, the pro-Russian president who was toppled in Kiev 10 days ago. Most people in the region are ethnic Ukrainians who speak Russian as their first language. Few now support Yanukovych, though many still look to Russia as an ally.
Pro-Russian demonstrations have been held in several eastern and southern cities since Saturday, in some cases ending with Russian flags raised at regional government buildings.
Kiev says Moscow has organized the demonstrations and sent hundreds of Russian citizens across the frontier to stage them.
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