America and its allies are ready to step up measures to "isolate Russia politically, diplomatically and economically" unless Moscow orders its troops in Crimea back to their barracks, US Secretary of State John Kerry has warned.
Mr Kerry's warning came as Prime Minister David Cameron said that Russia's failure to respect the territorial integrity and sovereignty of Ukraine was "unacceptable" and that any further incursion would be met by " further steps" from the international community.
Russian President Vladimir Putin insisted that it was "absolutely legitimate" for Moscow to use "all available means" to protect the Russian-speaking population of Ukraine - and left no doubt that this included the use of military force as a "last resort".
"Regarding the deployment of troops, the use of armed forces, so far, there is no need for it, but the possibility remains," said Mr Putin in a press conference held shortly after he ordered thousands of Russian soldiers on a military exercise near the Ukrainian border to return to their permanent bases.
Characterising Russia's actions as "humanitarian", Mr Putin accused the West of supporting an "unconstitutional coup" to oust Ukraine's legitimate president Viktor Yanukovych and warned that any punitive response from the international community would backfire by causing "mutual harm" to both Russia and its critics.
But Barack Obama said that Mr Putin was not "fooling anyone" and urged him to comply with international demands to de-escalate the crisis. "I think everyone recognises that, although Russia has legitimate interests in what happens in a neighbouring state, that doesn't give it the right to use force as a means of exerting influence inside of that state," said the US President.
Foreign Secretary William Hague said that Mr Putin's claims to be acting in response to an appeal for help from Mr Yanukovych and in defence of Ukraine's Russian-national population were "baseless".
In a statement to the House of Commons, Mr Hague said there was "a grave risk of escalation or miscalculation and a threat to hard-won peace and security in Europe". He commended the authorities in Kiev for their "refusal to be provoked" and urged them to maintain this position.
Mr Hague made clear he had no doubt that the troops in Crimea were acting on Moscow's orders, despite Mr Putin's insistence that they were "local self-defence units".
And he insisted that all of Britain's options "remain open" in response to the Russian actions, telling MPs that an inadvertently-revealed document which appeared to rule out trade sanctions was "not necessarily a guide to the decisions that will be made by Her Majesty's Government".
Mr Hague urged Russian ministers to meet with him tomorrow in Paris at a hastily-arranged conference to discuss the 1994 Budapest Memorandum, under which international powers including Russia gave assurances on the security and territorial integrity of Ukraine as it gave up its nuclear weapons. And he called on Moscow to agree to the deployment of international monitors from the UN and Organisation for Security and Co-operation in Europe (OSCE) to Ukraine.
Speaking during a visit to the Midlands, Mr Cameron said that his message to the people of Ukraine was that "we back your ability to choose your own future".
The Prime Minister added: "We... are sending a very clear message to the Government in Russia that what has happened is unacceptable.
"That failing to respect the territorial integrity and sovereignty of Ukraine: that is not an acceptable way to behave and we have said very clearly there will be costs and consequences diplomatically, politically and economically, from the decisions that have been taken.
"And we need to send an even more clear message that if there are further incursions into the Ukraine territorial integrity and sovereignty, then further steps would have to be taken."
Meanwhile Nato secretary general Anders Fogh Rasmussen said Moscow had agreed to convene an extraordinary meeting of the Nato-Russia Council tomorrow to discuss the situation under Article 4 of the Washington Treaty, which allows for consultations when the "territorial integrity, political independence, or security" of a member state is threatened.
The US agreed a one billion dollar (£600m) emergency loan guarantee to support heavily-indebted Ukraine, and Mr Kerry said Washington was looking at "a broader, more comprehensive plan".
Speaking after talks with the interim Ukrainian government in Kiev, Mr Kerry said Russia should abide by the 1997 agreement which grants it a naval base in the Crimean port of Sebastapol, but bars it from deploying troops outside this base.
"Russia, if it wanted to help de-escalate this situation could return its troops to the barracks, live by the 1997 base agreement and de-escalate, rather than expand, their invasion," said the Secretary of State.
"The United States of America would prefer to see this de-escalated. We would prefer to see this managed through the structures of legal institutions, international institutions that we have worked many years in order to deal with this kind of crisis.
"But if Russia does not choose to de-escalate, if it is not willing to work directly with the government of Ukraine as we hope they will be, then our partners will have absolutely no choice but to join us to continue to expand upon steps we have taken in recent days In order to isolate Russia politically, diplomatically and economically."
While Western nations "get" the historical context of Russia's desires, Mr Kerry said, "it is not appropriate to invade a country and, at the end of a barrel of a gun, dictate what you are trying to achieve. That is not 21st century, G8 major nation behaviour."
Tensions continued in Crimea, where warning shots were fired by Russian troops at Ukrainian soldiers who turned up at Beklek air base, singing the Ukrainian national anthem and demanding access to their workplace.
Mr Putin insisted that Russian troops in Crimea had been forced to step up security to defend their own bases against threats from "armed nationalists", but insisted that there was "no need" to deploy them elsewhere. And he added: "I proceed from the idea that we will not have to do anything of the kind in eastern Ukraine."
But he warned: "If we see... uncontrolled crime spreading to the eastern regions of the country, and if the people ask us for help, while we already have the official request from the legitimate President, we retain the right to use all available means to protect those people. We believe this would be absolutely legitimate. This is our last resort."