There are "indications" that Russia may be planning a military intervention in the south and east of Ukraine, Kiev's ambassador to the United Nations warns.
"There are indications that Russia is on its way to unleash a full blown military intervention in Ukraine's east and south," Yurii Klymenko said.
His statement was widely supported by other UN ambassadors, but challenged by a Russian diplomat, who read a prepared statement justifying Russia's actions so far.
President Obama used a press conference to underline the US commitment to support its Nato allies in the region. He also announced further sanctions against senior Russian government officials and new sanctions on individuals who provide support to the Russian leadership, and the Rossiya bank which many senior officials use.
A US official said the bank will be "frozen out of the dollar".
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Russia then announced its own sanctions against the USA, including barring named Americans including Senators John Boehner, John McCain and Harry Reid, from entering the country.
Meanwhile, EU leaders are meeting in Brussels to discuss extending travel bans and assets freezes against Russian political figures.
At a two-day summit in Brussels, starting on Thursday, British Prime Minister David Cameron will push for the EU to send a "clear warning" to Moscow, saying that the international community will pay a "very high price" if it fails to take action.
A strong and successful Ukraine is in all our interests. David Cameron
The EU has already implemented sanctions against 21 Russian and Ukrainian individuals in response to the referendum and annexation of Crimea. The US has also issued sanctions against 11 people.
However, Russian politicians have laughed off the sanctions – often saying that they do not have assets abroad and have no interest in travelling to the US and EU.
In a pre-summit phone call last night, Prime Minister Cameron and German Chancellor Angela Merkel agreed they would argue for the EU "building on" the travel bans and asset freezes already in place.
Following the call, a Downing street spokesperson said: "They reiterated that Sunday's referendum in Crimea and Russia's subsequent actions to annex Crimea are both illegal, and agreed that the EU should impose further consequences on Russia, building on the travel bans and asset freezes agreed by European foreign ministers on Monday.
"They also agreed that the EU should keeping working with the US and other international partners to de-escalate the situation and to deter Russia from embarking on any more action that could further destabilise Ukraine.
"They reaffirmed their support for the Ukrainian government and agreed that the international community should be doing all it can to provide economic assistance to the authorities in Kiev during this difficult time.
"The prime minister looks forward to the signing in Brussels on Friday of the political provisions of the association agreement between the EU and Ukraine.
"A strong and successful Ukraine is in all our interests and will show that efforts to undermine the country's independence and stability will not succeed."
It is expected that sanctions will extend beyond the existing list of 21 names.
'No war' with Russia
Meanwhile, United Nations Secretary General Ban Ki-moon met Russian President Vladimir Putin in Moscow. The UN was also deploying a 34-member human rights monitoring mission to Ukraine.
In the US, President Barack Obama ruled out a "war" with Russia, saying that it was only diplomatic resources that would be "mobilised".
"We are not going to be getting into a military excursion in Ukraine," he said. "What we are going to do is mobilize all of our diplomatic resources to make sure that we’ve got a strong international coalition that sends a clear message, which is the Ukraine should decide their own destiny.
"Russia right now is violating international law and the sovereignty of another country. Might doesn't make right. And, we are going to continue to ratchet up the pressure as it continues down its current course."
He added: "We need do not need to trigger an actual war with Russia."
In Crimea, the Ukrainian navy commander, Serhiy Haiduk, and several other hostages have been released, the Ukrainian presidential website said.
On Wednesday, a day of escalating military tensions in Crimea with the Ukrainian navy headquarters taken over by Russian troops, Admiral Haiduk was reported to have been driven what appeared to be Russian special forces.
Russian Defence Minister Sergei Shoigu asked the authorities in Crimea on Thursday to free the detained hostages and allow them safe passage out of the region.
19 March 2014
18 March 2014
17 March 2014
- Ukraine crisis: New face of Crimea revealed after naval base Independent
- Ukraine crisis: Naval bases seized in Crimea BBC News
- How to Punish Putin New York Times