UK Labour leader Jeremy Corbyn has refused to guarantee military support to another NATO nation if it was attacked by Russia.

He stressed the importance of building strong diplomatic relations with Moscow.

NATO’s article five enshrines the principle of “collective defence” where an attack against one member is considered an attack against all.

The UK, as a NATO member, would be expected to uphold that principle but Mr Corbyn would not give a concrete assurance that he would do so if he was prime minister.

Mr Corbyn was asked during the latest Labour leadership debate how he would react as PM to a violation by Vladimir Putin of the sovereignty of another NATO state. He said:

“You would obviously try to avoid that happening in the first place, you would build up a good dialogue with Russia to ask them and support them in respecting borders.”

The Labour leader repeatedly stressed the importance of improving diplomacy with Russia but would not firmly commit to upholding article five.

When pushed on whether he would sign off on the UK going to the aid of a NATO ally, he said:

“That’s in the NATO treaty. I would hope that we could strengthen our relationship and activity within the OSCE, the Organisation for Security and Co-operation in Europe, which includes Russia and every other state. We cannot allow a military build-up which is going to lead to some calamitous, incredibly dangerous situation.”

Mr Corbyn said he would want to avoid the UK entering into military action as he again stressed the importance of building diplomatic relationships. Pushed once more, he said:

“I don’t wish to go to war. What I want to do is achieve a world where we don’t need to go to war, where there is no need for it. That can be done.”

Leadership challenger Owen Smith was unequivocal in his response to the same question. He said:

“We would have to come to the aid of a fellow member of NATO. That’s the nature of the NATO accord. That would be the job of Britain in the event of a fellow NATO member being invaded, obviously. But it would be calamitous and we must never see that happen”.

Mr Smith also stressed the importance of improving diplomatic links between the UK and Russia.

Recently, US presidential candidate Donald Trump said he would not guarantee that the US would come to the aid of a NATO ally if it had not contributed 2% of its GDP to the organisation.

NATO members’ defence spending is supposed to amount to 2% of GDP, but only five out of 28 members met this target last year, according to a NATO report.