Ukraine crisis: Britain pulls out of G8 preparatory talks
The UK has pulled out of preparatory talks for the G8 summit because of Russia's build-up of troops in the Ukrainian region of Crimea.
The summit is due to be hosted by Russia in the city of Sochi.
Prince Edward, patron of the British Paralympic Association, has cancelled a visit to Russia for the Paralympic Games in Sochi, on government advice.
Downing Street said David Cameron and Barack Obama had spoken and agreed that Russia's actions were "unacceptable".
A spokesman said: "The prime minister and President Obama spoke at 9pm tonight [Sunday].
"They agreed that Russia's actions were completely unacceptable. They agreed on the urgent need for de-escalation and for Russia to engage in a dialogue directly with Ukraine.
"They agreed there must be 'significant costs' to Russia if it did not change course on Ukraine."
The spokesman said Mr Cameron was also speaking to Polish Prime Minister Donald Tusk and Lithuanian President Dalia Grybauskaite on Sunday evening.
Mr Cameron said British ministers would also now not attend the Paralympic Games.
British Paralympic Association chief executive Tim Hollingsworth told BBC Sport: "We completely understand the decisions that have been taken that mean UK ministers and His Royal Highness The Prince Edward will not attend the Sochi Paralympic Games.
"We are grateful for their continued full support for the British team to compete in Sochi. That remains our prime focus, the team are all in Sochi as of tomorrow [Monday] and training hard."
A Downing Street spokesman said Mr Cameron remained "fully supportive of our Paralympic athletes' participation at Sochi".
Earlier, Foreign Secretary William Hague flew to Kiev for talks with Ukraine's new leaders, amid rising tension with Russia.
Ukrainian acting president Olexander Turchynov has ordered a full mobilisation of Ukrainian military forces.
Mr Hague has said he will reiterate UK support for Ukraine's sovereignty.
Before leaving for Kiev, Mr Hague said: "We have to recognise the sovereignty and the territorial integrity of Ukraine has been violated and this cannot be the way to conduct international affairs.
"And so, in addition to calling yesterday's emergency meeting of the UN Security Council, the United Kingdom will join other G8 countries this week in suspending our co-operation under the G8, which Russia chairs this year, including the meetings this week for the preparation of the G8 summit."
He said Britain would keep its approach to further G8 meetings under review.
In Brussels, Nato held emergency talks about Russia's move to take control of Crimea.
Just before the meeting, Nato secretary-general Anders Fogh Rasmussen said Russia's troop deployments violated the principles of the United Nations charter and threatened peace and security in Europe.
"Russia must stop its military activities and its threats," he said.
The Ukrainian government has said it will seek the help of US and UK leaders to guarantee its security.
The interim prime minister, Arseniy Yatsenyuk, said Russia's military moves amounted to a "declaration of war".
On Saturday, President Obama held a 90-minute telephone conversation with Russian President Vladimir Putin and urged him to pull his forces back to bases in Crimea.
Mr Putin says Russia reserves the right to protect its interests and those of Russian speakers in Ukraine.
The Foreign Office says Mr Hague will make clear the UK's support for Ukraine's new government, which was formed after the ousting of former president Viktor Yanukovych in a popular uprising last month.
His removal from power followed four months of street protests that culminated in bloody clashes between demonstrators and security forces.
On Saturday, Mr Hague said the UK would work with international partners to "ensure that reforms by Ukraine are matched by international willingness to provide economic support".
He said Britain would also help Ukraine recover corruptly acquired assets.
BBC world affairs correspondent Emily Buchanan said Mr Hague was likely to urge the Ukrainian government not to do anything to provoke Russia, in order not to give Moscow a pretext for further military action.
Mr Hague said it was also important that Russia spoke to the Ukrainian authorities directly.
Russia, which rejects the legitimacy of Ukraine's new leaders, has so far refused to do so.
Former Conservative foreign secretary Sir Malcolm Rifkind said the situation was the "most serious crisis since the end of the Cold War".
"This needs to be a defining moment and Putin needs to understand this... in the West's relationship with Russia," he said.
A former British ambassador to Moscow, Sir Tony Brenton, said Mr Hague must "damp down" the tension between Russia and Ukraine.
He said: "[President Putin] has just seen his man, in effect, [Viktor] Yanukovych, overthrown by what he sees as a Western instigated revolt. He [Putin] is determined to maintain Russian influence in Ukraine as a whole, and he has now taken Crimea... hostage."
Labour leader Ed Miliband said there could be "no justification" for Russian military action in Ukraine.
"I believe diplomatic and economic pressure we can put on Russia is the best hope we can have for what everyone wants to see, which is the de-escalation of this crisis," he said.
In London, hundreds of Ukrainians staged two protests, one near the Russian embassy and the other at the Ukrainian embassy.
The demonstrators chanted slogans and held up banners that said "Hands off Ukraine". Similar protests were held in other European capitals.
The Foreign Office is advising against all travel to Crimea, and urging British nationals in Crimea to leave.
It said train and bus routes out of the peninsula were still operating, but it did not advise British nationals to try to leave by air from Simferopol until the situation had become more stable.