US President Barack Obama and British Prime Minister David Cameron vowed to stand together in support of Ukraine against Russia in a joint statement in The Times newspaper on Thursday.
"Russia has ripped up the rulebook with its illegal, self-declared annexation of Crimea and its troops on Ukrainian soil threatening and undermining a sovereign nation state," the two leaders wrote.
"With Russia trying to force a sovereign state to abandon its right to democracy and determining the course of its future at the barrel of a gun, we should support Ukraine`s right to determine its own democratic future and continue our efforts to enhance Ukrainian capabilities.
The op-ed article, issued shortly before the start of a NATO summit in Wales, said the alliance should build a "persistent" presence in Eastern Europe backed up by a rapid response force of land, air, sea and special forces that could "deploy anywhere in the world at very short notice".
Cameron and Obama urged fellow members of the 28-state alliance to meet their targets of spending at least 2 percent of GDP on military spending to show "our collective resolve is as strong as ever".
The op-ed said Britain and the United States would "not waver in our determination to confront" Islamic State extremists operating in Iraq and Syria, who have killed two US hostages and threatened to murder a Briton.
"If terrorists think we will weaken in the face of their threats they could not be more wrong. Countries like Britain and America will not be cowed by barbaric killers," the leaders wrote.