Russia has rejected reports that it threatened Ukraine with military assault if it does not surrender the Crimea by 3am on Tuesday as "total nonsense".
Amid the confusion of the worst diplomatic crisis since the Cold War, the Russian Defence Ministry told RT that the country has "become accustomed to the daily accusations by the Ukrainian media of carrying out some sort of military actions against our Ukrainian colleagues".
Relations between East and West have plummeted as the Russian Government continued to ignore calls from Western leaders to leave the Ukrainian area.
This morning, the Russian Foreign Minister Sergey Lavrov, justified the military incursion claiming it was necessary in order to protect his country's citizens living there. "This is a question of defending our citizens and compatriots, ensuring human rights, especially the right to life," he said.
It followed claims from the Ukrainian Defence Ministry that Russian fighter jets had "violated" the country's airspace over the Black Sea overnight and that Russian forces were now in control of the port city of Sevastopol, where the Black Sea fleet is based.
The takeover of the ferry terminal on the eastern tip of Crimea by Russian troops has exacerbated fears that Moscow is planning to bring even more troops into the strategic Black Sea region. There was no immediate comment from the Russian Defence Ministry.
In a series of interviews last night the US Secretary of State, John Kerry, said the Obama administration was looking at what sanctions to impose if Russia didn't "step back" from its military incursion. "There could even be, ultimately, asset freezes, visa bans" and disruption of trade, Kerry said on NBC's Meet the Press programme.
During a visit to Kiev today, the Foreign Secretary William Hague described the situation as "certainly the biggest crisis in Europe in the 21st century", adding that there was "no justification" for the actions in Crimea, where Russia had "in effect taken control".
Echoing US President Obama's speech on Friday, the Foreign Secretary said Moscow should be in no doubt that it faced "significant costs" for its move into the Ukrainian region.
However, at a news conference with Ukraine Prime Minister Arseniy Yatsenyuk, he said that any military response was not on the tale at the moment: "The UK is not discussing military options. Our concentration is on diplomatic and economic pressure."
President Vladimir Putin has defended Russia's actions, blaming "ultra-nationalist forces" for seizing power in Kiev. A Kremlin statement posted online said Putin told the German Chancellor, Angela Merkel, on Sunday that measures taken by Russia so far were "fully adequate" and in response to the unrelenting threat of violence" to "Russian citizens and the whole Russian-speaking population".
Earlier today, as Russia sought to draw support from the international community, Mr Lavrov said that he had discussed Ukraine with his Chinese counterpart and that the two veto-wielding UN Security Council members had "broadly coinciding points of view" and would stay in close contact on the issue.
Yesterday, Ukraine's country's acting Prime Minister said that his country was "on the brink of disaster" after hundreds of unidentified gunmen engaged in a stand-off with troops at a base in the Crimean peninsula.
Emerging from a closed session of parliament in Kiev, Mr Yatsenyuk described President Putin's decision to send in troops as a "declaration of war", and said "there was no reason for the Russian Federation to invade Ukraine.
The Nato Secretary-General, Anders Fogh Rasmussen, called on Russia to "stop its military activities and its threats" after convening an emergency meeting. "What Russia is doing now in Ukraine violates the principles of the United Nations Charter," he said. "It threatens peace and security in Europe."
Yesterday, the G7 group of major industrialised nations condemned Russia's intrusion into Ukraine . It has also cancelled preparations for the G8 summit that was to include Russia, scheduled for Sochi in June, the White House confirmed.
The group said in a statement: "We, the leaders of Canada, France, Germany, Italy, Japan, the United Kingdom and the United States and the President of the European Council and President of the European Commission, join together today to condemn the Russian Federation's clear violation of the sovereignty and territorial integrity of Ukraine.
"We have decided for the time being to suspend our participation in activities associated with the preparation of the scheduled G8 Summit in Sochi in June," it added.
The G7 also urged Russia to hold talks with Ukraine directly to address any human rights or security concerns it has.
The group expressed its support for Ukraine's bid to secure financial help from the International Monetary Fund and to make reforms to its economy.
The crisis has prompted a sell-off in Russian assets, with the rouble falling to an all-time low against the dollar at the opening of exchange trading on Monday.
Stocks also suffered globally, as traders became unnerved by the warnings from Washington and other governments that Moscow, an oil exporter, might face sanctions.
Tensions have been rising since President Yanukovych was ousted last week, following months of pro-EU protests.
Mr Yanukovych fled to Russia last Saturday after more than 80 people were killed in the protests, but insists that he is still President.
Additional reporting by Reuters and AP
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