Hague urges Ukraine 'not to rise to Russian provocation'
There is a risk that "deliberate provocation" could give rise to a dangerous incident in Ukraine, Foreign Secretary William Hague has warned.
The government of Ukraine should be commended "for refusing to rise to provocation" from Russia, he told MPs.
"I urged them yesterday to maintain that posture through all circumstances and at all times," he added.
The UK's ambassador to the UN, Sir Mark Lyall Grant, has condemned Russia's presence in Crimea.
There have been no reported incidents since the 03:00 GMT deadline allegedly set by Russia for Ukrainian forces in Crimea to surrender or face attack.
Russia has denied issuing any ultimatum to Ukraine, but Vladimir Putin has said Russia reserves the right to protect its interests.
Its is being reported that Russian troops have fired warning shots over the heads of Ukrainian soldiers.
Asked in the Commons whether this could lead to a "conflagration", Mr Hague replied: "This continues to be a serious risk, particularly that deliberate provocation could give rise to a dangerous incident.
"I commend the Ukrainian authorities for refusing to rise to provocation, and I urged them yesterday to maintain that posture through all circumstances and at all times, and I believe they will be determined to do so."
His comments came as Kiev's UN envoy Yuriy Sergeyev said Russia had deployed about 16,000 troops to Crimea.
An official document, photographed as a senior official walked along Downing Street holding it on Monday, said the UK government should not curb trade with Russia or close London's financial centre to Russians as part of any possible package of sanctions against Moscow.
The same paperwork appeared to confirm that ministers are considering - along with other EU countries - visa restrictions and travel bans on key Russian figures.
It also said 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".
But Mr Hague insisted that the document did not fairly summarise government policy.
"Any such photographing or indeed making documents available for photographing is absolutely regrettable and should not happen. I hope all officials will ensure in future it does not happen," he told MPs.
"Nevertheless, it should also be seen in perspective... I want to make it absolutely clear that anything that is written in one document that is being carried by one official is not necessarily any guide to the decisions that will be made by Her Majesty's government, and our options remain very much open on this subject."
Labour's shadow foreign secretary Douglas Alexander said the incident had been "a very serious blunder at a very serious time".
The leak "risks compromising the UK's influence with both Russia and indeed her key allies", he argued.
Prime Minister David Cameron said on Monday that diplomatic, political and economic pressures would be put on Russia to send a "clear message" about its actions in the Ukraine.
Earlier, Mr Hague had said the situation was the "biggest crisis" facing Europe in the 21st Century and called for immediate dialogue between Russia and Ukraine.
Former Conservative Defence Secretary Liam Fox told BBC Radio 4's World at One: "We have to make very clear at all times that sovereign nations have to be able to exercise their self-determination free from hindrance and interference.
"There is a pattern of behaviour that has been established, a very thuggish pattern of behaviour by the Kremlin, bullying the Ukraine through gas, the invasion of Georgia where they still have troops in South Ossetia and Abkhazia, the attitude they had towards Syria."
US Secretary of State John Kerry is in Kiev for talks amid speculation that the US could unilaterally impose tough measures.
'Ignoring history's lessons'
BBC political editor Nick Robinson said there had been an "enormous gap" in the rhetoric between Mr Kerry, who has appeared on TV listing a series of possible sanctions the US might try to impose on Russia, and Mr Hague, who has been reluctant to spell out any specifics.
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 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.
The Foreign Office has advised against all travel to Crimea, and urged British nationals in the peninsula to leave.