Britain and other western states are piling pressure on Vladimir Putin's Russia as the military standoff in the Crimea continues.
Intense diplomacy is taking place around the world as tensions continue to rise in the Crimea, which remains dominated by Russian military forces.
Foreign secretary William Hague warned Russia's actions were "clearly… a violation of the sovereignty and territory integrity of Ukraine".
He said European foreign ministers are discussing what "significant costs" Russia will have to pay for the crisis.
"If Russia continues on this course we have to be clear this is not an acceptable way to conduct international relations," he told the Today programme.
"There will be significant costs to that. The world cannot just allow this to happen. The world cannot say it's OK to violate the sovereignty of another nation in this way."
Downing Street has made clear Britain is not considering military action.
Hague is in Kiev, where he will set to meet Ukraine's new president Olexander Turchinov and prime minister Arseny Yatsenyuk.
He will also visit Independence Square, whose Maidan protesters toppled Viktor Yanukovych's government and remains deeply suspicious of the new Ukrainian administration.
Hague praised the Ukrainian authority for refusing to rise to provocation from the Russians, but warned of a "constant risk of a miscalculation, of a flashpoint occurring".
Intense diplomatic activity is underway at the UN security council, which has already held an emergency meeting, and in the G7.
Britain and the other G7 states have withdrawn from preparations for this year's G8 summit, which is to be held in Sochi.
The suspension of involvement in preparations for the summit will stand "until the environment comes back where the G8 is able to have meaningful discussion", a statement released by the White House on Sunday evening said.
Britain's ministers are boycotting the Sochi 2014 Paralympic Games. Prince Edward, the patron of the British Paralympic Association, will not be attending the Games.
Meanwhile European foreign ministers are meeting in Brussels to discuss the EU's response to Russia's move.
Speaking before the meeting, Europe minister David Lidington said: "If there are genuine concerns about the treatment of minorities or the status of the Russian language, there is the way that those concerns can be addressed, and I fear the refusal so far of the Russian government to engage in that sort of direct contact with the Ukrainian government is deeply concerning."
The early diplomatic moves against Russia do not appear to have dented Putin's resolve to consolidate control over the Crimea.
Reports from the province, which was Russian until 1954, suggest more Russian troop and ship movements are underway.
Crimean airports have been secured by the Russian military and two large Ukrainian military bases are completely surrounded.
The fledgling Ukrainian government has ordered a full military mobilisation in response.