Meeting NATO partners the day after Trump revealed plans to start peace talks with Russia's Vladimir Putin, the Pentagon head said Washington remained committed to the alliance but members needed to do more for it to "endure".

"Our partners must do far more for Europe's defence. We must make NATO great again," Hegseth told reporters after the talks in Brussels, warning that "President Trump will not allow anyone to turn Uncle Sam into Uncle Sucker."

The former Fox TV contributor and military veteran described the Ukraine conflict as "a factory reset for NATO", echoing Trump's demands for allies to more than double their defence spending targets to five percent of GDP.

"There is no replacement for hard power," he said, after warning that confronting Russia was an "important European responsibility".

The United States has underpinned European security through NATO over the past seven decades.

Yet, when asked whether the United States, which currently spends around 3.4 percent of GDP on defence, planned to reach the five percent target itself Hegseth was noncommittal.

"Ultimately, we have our own budgetary considerations to be had," he said.

Addressing concerns over red lines on Ukraine that he outlined on Wednesday -- including that Kyiv's aim of returning to its pre-2014 borders was "illusionary" -- Hegseth denied they amounted to "concessions" to Putin.

"It's a recognition of the hard power realities on the ground," he said.