Ukraine's military says Russia has given its forces an ultimatum to surrender in Crimea or face an all-out assault on the strategic Black Sea peninsula that has been overrun by Kremlin-backed troops.
But Russia's Black Sea fleet swiftly denied any such demand and the country's parliament speaker said there was no need yet for Moscow to use its right to launch military action in Ukraine.
Regional Ukrainian Defence Ministry spokesman Vladyslav Seleznyov said in the Crimean capital Simferopol the ultimatum is to recognise the new Crimean authorities, lay down their weapons and leave, or be ready for an assault.
Pro-Russian authorities in Crimea will cut off water and electricity to Ukrainian soldiers in bases surrounded by Russian forces on Monday night, a Russian former lawmaker loyal to President Vladimir Putin said.
Sergei Markov, who held meetings with pro-Russian authorities on the Ukrainian peninsula earlier on Monday, said the soldiers would also be told they would not receive their next pay packet if they did not publicly renounce their loyalty to the new provisional government in Kiev.
"If they stay here and remain loyal to Kiev and the Ukrainian government, it will become more uncomfortable for them," said Mr Markov, who sits in a Kremlin-backed public policy chamber. "The pressure is going to increase tonight."
Ukraine has accused Russia of pouring more troops into Crimea as world leaders grappled with Europe's worst standoff since the Cold War.
Russia has deployed roughly 16,000 troops to the region since last week, Kiev's UN ambassador Yuriy Sergeyev said at an emergency meeting of the UN Security Council.
Ukraine border guards said that Russian forces began moving troops into Ukraine's Crimea region by ferry on Monday after seizing control of the border post on the Ukrainian side of the waterway. Russians who seized the isolated Black Sea peninsula have been surrounding the ferry terminal for days but until now had not taken control of Ukraine's border guard station.
A border guard spokesman said Russian troops seized the checkpoint after the border guards tried to stop two buses carrying seven armed men, and the next ferry brought three truckloads of soldiers across.
Acting Ukrainian President Oleksander Turchynov said earlier that 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," he said. "I appeal to Russia's leadership - stop the provocative actions, aggression and piracy. This is a crime and you will answer for it."
Meanwhile Ukraine's ousted leader Viktor Yanukovych has sent a letter to Mr Putin requesting that he use the Russian military to restore law and order in Ukraine, Moscow's UN envoy said on Monday.
"Under the influence of Western countries, there are open acts of terror and violence," Russian ambassador Vitaly Churkin read from the letter in the emergency meeting of the UN Security Council, which was called by Russia.
"People are being persecuted for language and political reasons," he quoted the letter as saying. "So in this regard I would call on the President of Russia, Mr Putin, asking him to use the armed forces of the Russian Federation to establish legitimacy, peace, law and order, stability and defending the people of Ukraine."
Talks of an ultimatum come as the European Union threatens to freeze visa liberalisation and economic co-operation talks and boycott the G8 summit in Russia's Sochi if Moscow does not de-escalate the crisis in Ukraine's Crimean peninsula by Thursday.
French Foreign Minister Laurent Fabius said the EU would give Russia until an emergency summit of EU leaders is held on Thursday to show clear signs of goodwill, including a willingness to open talks and a withdrawal of Russian troops to their barracks in Crimea. If not, Mr Fabius said the EU would start implementing punitive measures.
EU Council president Herman Van Rompuy called the emergency summit late on Monday after the foreign ministers ended their talks.
US President Barack Obama said on Monday that Russia has violated international law in its military intervention in Ukraine and said the US government has warned it will look at a series of economic and diplomatic sanctions that would isolate Moscow.
Mr Putin needs to allow international monitors to mediate a deal in Ukraine acceptable to all Ukrainian people, Mr Obama said.
"Over time this will be a costly proposition for Russia. And now is the time for them to consider whether they can serve their interests in a way that resorts to diplomacy as opposed to force," Mr Obama said.
Reuters reported that the US is likely "moving down the path" of imposing sanctions against Russia if actions in Ukraine proceed.
Canadian Prime Minister Stephen Harper said Russia could ultimately be forced out of the G8. "President Putin's actions have put his country on a course of diplomatic and economic isolation that could well see Russia exit the G8 entirely," he said.
US Secretary of State John Kerry also "put at question Russia's capacity to be within the G8", which is comprised of the US, Canada, Britain, Germany, France, Italy, Japan and Russia.
The rising threat of war between Ukraine and Russia pushed global stocks down sharply - the Moscow market fell 11.5 per cent - and lifted gold to a four-month high.
Fears of further destabilisation in Europe have prompted Poland, a neighbour of Ukraine, to convene discussions with NATO.
NATO will hold emergency discussions on Ukraine on Tuesday after Poland requested consultations under Article 4 of the alliance's treaty, NATO said on Monday.
Under Article 4, any ally can request consultations whenever, in the opinion of any of them, their territorial integrity, political independence or security is threatened.
"The developments in and around Ukraine are seen to constitute a threat to neighbouring Allied countries and having direct and serious implications for the security and stability of the Euro-Atlantic area," the alliance said.
AP, AFP, Reuters