Addressing a meeting of Kyiv's backers in Brussels, Hegseth laid out President Donald Trump's red lines and demands -- and told Europe to ramp up both support for Ukraine and spending on its own defence.

"To be clear, as part of any security guarantee, there will not be US troops deployed to Ukraine," he said.

The US defence chief said any peace process "must start by recognising that returning to Ukraine's pre-2014 borders is an unrealistic objective" -- although Washington, like Europe, wanted "a sovereign and prosperous Ukraine".

And he warned that "the United States does not believe that NATO membership for Ukraine is a realistic outcome of a negotiated settlement".

Washington's allies have been waiting nervously for clarity from Trump's administration after the volatile leader demanded that NATO more than double its defence spending target and vowed to end the war in Ukraine.

Hegseth's two days of talks in Brussels are part of a flurry of visits to Europe by top US officials, culminating with Vice President JD Vance meeting Ukrainian President Volodymyr Zelensky at a security conference in Munich on Friday.

On both support for Kyiv and Europe's own defence spending, the Pentagon chief delivered the resounding message that Washington expects its allies to do more.

"Safeguarding European security must be an imperative for European members of NATO," Hegseth said. "Europe must provide the overwhelming share of future lethal and non-lethal aid to Ukraine."

Hegseth further warned that Washington would not accept what it considers "imbalanced" burden-sharing within the transatlantic alliance.

"The United States remains committed to the NATO alliance and to the defence partnership with Europe, full stop," he said.

"But the United States will no longer tolerate an imbalanced relationship which encourages dependency."