Ukraine: Obama says Russia 'acting out of weakness'
US President Barack Obama has said Russia is acting "not out of strength, but out of weakness" in Ukraine.
He also warned of the possibility of further sanctions against Russia if it encroached further into Ukraine.
Speaking in The Hague, Mr Obama said he was encouraged by the willingness of EU countries to consider sanctions that could possibly hurt them as well.
The US has already imposed sanctions on Russian individuals after Moscow annexed Crimea earlier this month.
Mr Obama, who is attending a meeting of G7 countries, said the annexation was not a "done deal" as the international community had not recognised it but admitted that "the facts on the ground are that the Russian military controls Crimea".
He expressed concern about the large numbers of Russian troops massed on the border but did say Russia had the right to deploy troops on its own territory.
"It is up to Russia to act responsibly and show itself once again to be willing to abide by international norms and... if it fails to do so, there will be some costs," he said.
Mr Obama said that certain sanctions discussed with European countries could "cause some disruptions to each of our economies, or certain industries" but said he was "encouraged by... the firmness and willingness on the part of all countries to look at ways in which they can participate in this process".
He added that he hoped the International Monetary Fund would quickly finalise an aid package for Ukraine, adding that it was important to help Ukraine hold successful elections in May.
Also on Tuesday evening, there were reports that Russian troops had seized the last remaining Ukrainian warship in Crimea.
A Ukrainian defence spokesman said a Russian-flagged tugboat was towing away the minesweeper Cherkassy.
The last military base under Ukrainian control in the region, in Feodosia, was seized by Russian forces on Monday.
On Monday the G7 group of industrialised countries issued a statement condemning Russia's actions and excluding it from what was the G8.
Concerns about possible tougher sanctions have been adding to worries over Russia's economy, the BBC's Steve Rosenberg reports from Moscow.
According to the country's deputy economy minister, capital flight this year is expected to reach $70bn (£43bn) by the end of March, meaning more money has left Russia in the last three months than in the whole of 2013, our correspondent reports.
Some foreign investors now think Moscow seems to be prioritising geopolitics and nationalistic fervour over stable markets, he adds.
On Monday the head of Russia's largest bank, the state owned Sberbank, warned that Russia was now at risk of recession.
The possibility of sanctions against Russian energy imports has been raised, with some calling for the US to ease its current restrictions on gas exports.
On Tuesday, Lithuania's energy minister called on the US Senate to speed up the export of natural gas to Europe.
Jaroslav Neverovic said that Lithuania was being forced to pay a "political price" for being entirely dependent on Russian gas supplies.
Also on Tuesday, an Ukrainian ultra-nationalist leader was shot dead in what officials described as a special forces operation.
Oleksandr Muzychko, better known as Sashko Bily, died in a shoot-out with police in a cafe in Rivne in western Ukraine, the interior ministry said.
He was a leader of Right Sector, a far-right group which was prominent in the recent anti-government protests.
Ukraine's Deputy Interior Minister Vladimir Yevdokimov said Muzychko died after opening fire at police and Sokol special forces, who had raided a cafe to arrest him and fellow ultra-nationalists. The authorities described Muzychko as a criminal gang leader.
However, a Ukrainian MP, Oles Doniy, gave a different version of events. He said two cars had forced Muzychko's car to stop, and he had then been dragged into one of the other cars.
Later his body was found dumped, his hands tied behind his back and two bullet wounds in his heart, Mr Doniy wrote overnight on his Facebook page.
A Right Sector organiser in Rivne has now threatened revenge for the killing of Muzychko, saying he had not been summoned by investigators.
Moscow says the activities of Right Sector and other Ukrainian nationalist groups pose a threat to the large Russian-speaking minority in Ukraine. President Vladimir Putin gave that as one of his reasons for intervening in Crimea.
However, some commentators say Russia has deliberately whipped up such fears, and that the influence of Right Sector in Ukrainian politics is exaggerated.