BRUSSELS: US President Barack Obama urged the West to remain united in its response to Moscow's takeover of Crimea, saying Wednesday that “with time” Russians will realise that “brute force” cannot win.

“With time, so long as we remain united, the Russian people will recognize that they cannot achieve security, prosperity and the status they seek through brute force.

“And that's why throughout this crisis we will combine our substantial pressure on Russia with an open door for diplomacy,” Obama said in his only speech in a six-day tour of Europe.

Obama told a crowd of some 2,000 people massed in a concert-hall with Belgian's king and queen that Russia's actions had challenged “truths that only a few weeks ago seemed self-evident”, that international law matters, that borders cannot be redrawn, that people must decide their own future.

Obama said that though the West was not directly affected by Russia's incursion “indifference would ignore the lessons that are written in the cemeteries of this continent”.

Earlier in the day he toured the only World War I cemetery in Belgium to hold US graves.

“This is not another Cold War we're entering into,” he said of the Ukraine crisis.

“The United States and Nato do not seek any conflict with Russia.”

But he said that the transatlantic military alliance would uphold its “one-for-all” duty to defend each other's sovereignty.

“And in that promise we will never waiver.

“Nato nations never stand alone,” he added, saying that the alliance was patrolling the skies over the Baltics, and had reinforced its presence in Poland. “And we're prepared to do more.”