Austin's visit came at a time when Ukrainian President Volodymyr Zelensky is urging Western allies to lift restrictions on the the use of long-range weapons and calling for an immediate invitation to join NATO.

But Ukraine fears the prospect of reduced US military support should Donald Trump be elected to the White House next month.

"I'm back in Ukraine for the fourth time as Secretary of Defense," Austin wrote on X, posting a picture of himself at the railway station.

He said his previously unannounced visit demonstrated that "the United States, alongside the international community, continues to stand by Ukraine".

The US Department of Defense said Austin would "meet with Ukrainian leadership" and "underscore the US commitment to providing Ukraine with the security assistance it needs to defend itself from Russian aggression on the battlefield".

Austin will also deliver a speech, it said.

US President Joe Biden on Friday along with the leaders of Germany, France and Britain pledged to keep up support for Ukraine and condemned "Russia's continued war of aggression".

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