Ukraine crisis: UK warns Russia over Crimean incursion
Diplomatic, political and economic pressures will be put on Russia to send a "clear message" about its actions in the Ukraine, David Cameron has said.
The prime minister was speaking after Russian forces reportedly set a 03:00 GMT deadline for Ukrainian forces in Crimea to surrender or face attack.
The international community has warned Russia of reprisals if it does not pull its forces back and reduce tensions.
Vladimir Putin has said Russia reserves the right to protect its interests.
Mr Cameron spoke to German Chancellor Angela Merkel and French President Francois Hollande about the crisis on Monday.
Downing Street said they had agreed they "should speak with one voice and send a clear message to Russia that its actions in Ukraine were completely unacceptable".
Meanwhile, an official document says the UK government will not curb trade with Russia or close London's financial centre to Russians as part of any possible package of sanctions against Moscow.
The document, which was photographed as a senior official carried it into a meeting in Downing Street, says "the UK should not support for now trade sanctions or close London's financial centre to Russians".
The same paperwork confirms that ministers are considering - along with other EU countries - visa restrictions and travel bans on key Russian figures.
It also says that ministers should "discourage any discussion (eg, at Nato) of contingency military preparations" and support "contingency EU work on providing Ukraine with alternative gas" and oil supplies "if Russia cuts them off."
Mr Cameron, who chaired a meeting of the National Security Council on Monday on the UK's response, said Russia's actions would have "costs and consequences" and Ukraine should be allowed to decide its political future.
The prime minister said reports of a Russian ultimatum - which Moscow has denied - were "deeply concerning".
"What we want to see is a de-escalation rather than a continuation down the path that Russia has taken, violating the sovereignty and territorial integrity of another country," he said.
"So we shall have to bring to bear diplomatic, political, economic and other pressures in order to make this point. That is the very clear message the whole world needs to send to the Russian government."
Ukraine has ordered a full military mobilisation in response to Russia's build-up of forces in Crimea, which was part of Russia until 1954.
No 10 has ruled out military intervention, saying the "only avenue that is being pursued is a diplomatic and peaceful one", but warned of a series of potential measures against Russia.
Speaking in Kiev, Foreign Secretary William Hague said the situation was the "biggest crisis" facing Europe in the 21st Century and called for immediate dialogue between the two nations.
Mr Hague, who travelled to Kiev on Sunday to hold talks with the new Ukrainian government, told the BBC that Russia had "legitimate" interests in the region.
But he said its actions were unacceptable and required a "strong" response from the international community.
"It is a very tense and dangerous situation that Russia's intervention has now produced," he told BBC Radio 4's Today, adding that there was now a constant risk of a "flashpoint".
"The world cannot just allow this to happen. The world cannot just say it is OK, in effect, to violate the sovereignty of another nation in this way."
Mr Hague rejected claims the US and EU were powerless to act, saying they had a range of options at their disposal if Russian forces did not return to their naval bases in Crimea and honour the terms of an agreement with Ukraine allowing them to station forces there.
The UK has said Russia will face "significant costs" if it does not pull back, with economic action not being ruled out.
Speaking after meeting Ukraine's interim president and prime minister, Mr Hague said Russia's actions could not be allowed to become the "normal way to behave in international affairs".
The foreign secretary said there was a "serious threat to G8 co-operation in the coming weeks and months".
The UK and other G7 nations have already said they are suspending preparations for this year's G8 summit in Russia, while the US has hinted at stronger measures, such as possible sanctions and asset freezes on Russian business.
The G7, which comprises the UK, US, Canada, France, Germany, Italy and Japan, has called on Moscow to "address any ongoing security or human rights concerns that it has with Ukraine through direct negotiations".
It has also repeated its commitment to provide substantial financial backing to the new Ukrainian government, with talks with the International Monetary Fund due to begin in the coming days.
The UK, itself, has said it will give £10m to Kiev to support economic and political reforms.
The Russian incursion was triggered by former Ukrainian leader Viktor Yanukovych's removal from power following four months of street protests that culminated in bloody clashes between demonstrators and security forces.
Former Labour foreign secretary Jack Straw told the BBC that Mr Hague - who is expected to make a statement to Parliament on the crisis upon his return on Tuesday - was right to proceed with "great caution".
"One third of Germany's gas comes from Russia so the idea of economic sanctions could cut both ways," he told BBC Radio 4's World at One.
He added: "You have to do something...but I also say with the benefit of experience that 'that something' has to be pretty carefully calibrated so it does not make matters worse or just make the West look like a paper tiger."
The Foreign Office has advised against all travel to Crimea, and urged British nationals in the peninsula to leave.