The conflict in eastern Ukraine is not NATO's responsibility despite Russia's "violation of international law", the alliance's Secretary General has said.

Jens Stoltenberg also said the UK must maintain its military contributions to the organisation, which is set at 2% of GDP.

But, asked if NATO would do anything stop Vladimir Putin's forces in Ukraine, he said: "Our main responsibility is to defend and to protect all NATO allies and no NATO ally has been attacked," he told Sky News.

"So deterrence is working because everyone sees that NATO is a very strong alliance and we are now implementing the biggest reinforcement of our collective defence since the end of the Cold War.

"Ukraine is not a member of NATO, so there is a difference between Ukraine and NATO members, because the security guarantee is for NATO allies.

"But we don't respect or accept in any way what Russia is doing in Ukraine and that's the reason why we provide strong political support for Ukraine.

"The most important thing now is to support the implementation of the Minsk agreements, meaning respecting the ceasefire, making sure that all their weapons are withdrawn from the frontline and to make sure that happens we need proper monitoring."

Asked about military leaders' concerns that Britain may not maintain defence spending at the target of 2% of GDP, Mr Stoltenberg said: "At the same time I know that Britain is next to the US as the country that invests most in defence.

"I count on the UK to do so as I count on all other allies to implement and honour their pledges."

Mr Stoltenberg's comments are likely to be seen as further evidence of the fears within the alliance that none of the main political parties has committed to meet the target beyond the next financial year.