An Australian war expert has expressed grave concerns Russia's invasion of Ukraine could escalate to a European war involving nuclear weapons.

Paul Dibb, a former Defence department deputy secretary for strategy and intelligence, wrote a report that says the risk of nuclear war is greater than at any time since the Cold War.

He told Liam Bartlett on 6PR Mornings the Australia-US joint facility at Pine Gap could become "a priority target" of both Russia and China.

"What we've got now is an unprecedented attack across an internationally-recognised boundary, Ukraine," he told Bartlett.

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