Russian President Vladimir Putin could view the prospect of defeat in Ukraine as an existential threat to his regime, potentially triggering his resort to using a nuclear weapon, a top US intelligence official has warned, The Guardian reported.

The warning came in an assessment from intelligence chiefs briefing the Senate on worldwide threats.

The Director of National Intelligence, Avril Haines, told the Senate's Armed Services Committee that Putin would continue to brandish Russia's nuclear arsenal in an attempt to deter the US and its allies from extending further support for Ukraine.

The shift of focus to the east and the south are most likely a temporary tactic rather than a permanent scaling back of war aims, she said, The Guardian reported.

The Russian leader would not use a nuclear weapon until he sees an existential threat to Russia or his regime, Haines argued.

But she added that he could view the prospect of defeat in Ukraine as constituting such a threat, the report said.

"We do think that [Putin's perception of an existential threat] could be the case if he perceives that he is losing the war in Ukraine and that Nato in effect is either intervening or about to intervene in that context, which would contribute to a perception that he is about to lose the war in Ukraine," Haines told the committee hearing.

She added that the world would probably have some warning that nuclear use is imminent, The Guardian reported.

The prediction for Ukraine is a long, grueling war of attrition, which could lead to increasingly volatile acts of escalation from Putin, including full mobilization, the imposition of martial law, and if the Russian leader feels the war is going against him, endangering his position in Moscow even the use of a nuclear warhead, the report said. (AA/IANS)