Speaking at a meeting of Ukraine's military allies at NATO headquarters in Brussels on Wednesday, Hegseth delivered the clearest and bluntest public statement so far on the new U.S. administration's approach to the nearly three-year-old war.

"We want, like you, a sovereign and prosperous Ukraine. But we must start by recognising that returning to Ukraine’s pre-2014 borders is an unrealistic objective," Hegseth told the meeting of Ukrainian officials and more than 40 allies.

"Chasing this illusionary goal will only prolong the war and cause more suffering," he added.

He also told Washington's NATO allies that they would have to step up and assume greater responsibility for Europe's security. He said "stark strategic realities", such U.S. border security issues and threats posed by China, prevented the U.S. from being "primarily focused on the security of Europe".

His remarks on Ukraine were a stark change from the stance of the Biden administration and many of Ukraine's closest allies, who had declared they would support Kyiv for as long as necessary and stressed the importance of territorial integrity.

His comments also suggested that Kyiv would have to abandon some of its key war aims - regaining territory from Russia and securing protection from future attacks through membership of the U.S.-led NATO military alliance.

While Trump administration officials had signalled for some time that they would not support those goals, Hegseth's on-camera remarks made that stance clear to the global public.

Though Ukraine set its sights on ejecting all Russian troops from its territory for much of the war, it has increasingly acknowledged that retaking its land by force is unfeasible and that diplomacy is a more realistic course of action.

The Ukrainian foreign ministry did not immediately respond to a request for comment.

NO US TROOPS IN UKRAINE

Hegseth said any durable peace must include "robust security guarantees to ensure that the war will not begin again". But he said U.S. troops would not be deployed to Ukraine as part of such guarantees.

He also said "the United States does not believe that NATO membership for Ukraine is a realistic outcome of a negotiated settlement".

Instead, security guarantees should be backed by "capable European and non-European troops", the Pentagon chief said.

"If these troops are deployed as peacekeepers to Ukraine at any point, they should be deployed as part of a non-NATO mission and they should not be covered under Article 5," he said, referring to the alliance's mutual defence clause.

Russia annexed the Black Sea peninsula of Crimea from Ukraine in March 2014 and then backed pro-Russian separatists in an armed insurgency against Kyiv's forces in the eastern Donbas region of Ukraine.

Moscow currently controls about 20% of Ukraine's territory, mainly in the east and south.