US president Barack Obama has dismissed Russia as a "regional power" that threatens its immediate neighbours "not out of strength but out of weakness".
Following the West's suspension of its 16-year collaboration with Russia in the G8 group of leading industrialised nations, Obama said that Moscow was "more isolated than [it was] during most of the 20th century when it was part of the Soviet Union".
Leaders from the US, UK, France, Germany, Italy, Canada and Japan issued a joint statement in The Hague saying they would not attend the planned G8 meeting in the Russian resort town of Sochi and would boycott the April G8 in Moscow.
They also threatened further economic and diplomatic sanctions should Russia invade eastern Ukraine.
The US president said that the Crimean unification with Russia "is not a done deal in the sense that the international community, by and large, is not recognising the annexation of Crimea".
But he said there was no expectation that Russian forces would be kicked out of the Black Sea peninsula by force.
"Obviously, the facts on the ground are that the Russian military controls Crimea [and] there are a number of individuals inside of Crimea that are supporting that process," he added.
"[The US] generally doesn't need to invade our neighbours to have influence on them," he said. And in a sideswipe against the Putin regime, he called Russia a "regional power".
He also rejected Putin's comparison of Crimea to Kosovo as nonsense.
"Thousands were being killed in Kosovo by their government," he said.
"It is up to Russia to act responsibly and show itself once again to be willing to abide by international norms and if it fails to do so, there will be costs," Obama added.
He was speaking at a news conference with Dutch prmme Minister Mark Rutte at the end of the Nuclear Security Summit.