U.S. President Barack Obama

U.S. President Barack Obama has urged NATO leaders on Friday to stand firm against a resurgent Russia over its seizure of Crimea from Ukraine, saying Britain’s vote to leave the European Union should not weaken the western defence alliance.

The special relationship between the US and the UK will endure. I have no doubt that the UK will remain one of NATO’s most capable members,” he said, adding that the vote raised significant questions about the future of EU integration.

The 28-nation NATO alliance will formally agree on Friday to deploy four battalions with 3,000 to 4,000 troops in the Baltic States and eastern Poland on a rotating basis to reassure eastern members of its readiness to defend them.

In Warsaw, we must reaffirm our determination, our duty under Article 5 of the North Atlantic Treaty to defend every NATO ally,” Obama said, saying the West must help Ukraine defend its sovereignty and territorial integrity by keeping sanctions on Moscow until it fully complies with a ceasefire agreement.

We need to bolster the defence of our allies in central and eastern Europe, strengthen deterrence and boost our resilience against new threats, including cyber-attacks.”

Estonia, Latvia, Lithuania and Poland requested a permanent NATO presence amid fears that Moscow could seek to destabilize their pro-Western government through cyber-attacks, stirring up Russian speakers, hostile broadcasting and even territorial incursions.

Critics say the NATO plan is a minimal trip wire that might not deter Russian action.

The Kremlin denies any such intention and says NATO is the aggressor by moving its borders ever closer onto former Soviet territory which it regards as its sphere of influence.