A former UK security chief has warned Vladimir Putin could strike a NATO base to stop the transfer of weapons to Ukraine. 

Lord Peter Ricketts, who was the British government's first national security adviser, said the Russian President is becoming "increasingly desperate to choke off the flow of arms" to Ukraine.

He said Putin wants the world to fear that he will use nuclear weapons against Ukraine.

"I suppose what President Putin wants us all to fear is that he might want to use a nuclear weapon in Ukraine, which would be a momentous escalation and ensure Russia was isolated around the world," he told the BBC.

"More likely, I think what they're looking at is some way of preventing or limiting this flow of arms into Ukraine, keeping the Ukrainian Armed Forces going.

"So we may see attacks on convoys or aircraft bringing the arms in from the west."

Lord Ricketts told the BBC the way Putin might stop the transfer of weapons to Ukraine is by attacking aircraft or convoys headed to the country from NATO.

"At worst, possibly, some kind of missile strike on a base in NATO, where the arms for Ukraine were being prepared," Lord Ricketts said. 

"And that will certainly pose the NATO countries with a real dilemma."

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Lord Rickets was worried the Ukraine invasion is moving towards "an angry stalemate, where Russia will hold on to quite a large part of the country" and Ukraine will not agree to a settlement deal.

He believed the invasion could continue for "years and years to come" as a guerrilla war like an "open wound in the middle of Europe".