The United States began reviewing its military presence in Europe on Monday, officials said, after US Defense Secretary Pete Hegseth told his NATO counterparts last month that Washington was planning such an analysis.

"As the Secretary (Hegseth) laid out then, this will be a real review, designed to ensure that NATO is moving fast and irreversibly toward Europe taking primary responsibility for its conventional defense," US Defense Under Secretary Elbridge Colby wrote on X.

"The outcome of this Review will be an acceleration of NATO's transition to a stronger, more equitable, and sustainable alliance," Colby added.

President Donald Trump has repeatedly lashed out against European allies over their response to his war in Iran, including the refusal to allow US forces to use their bases during the start of the conflict.

Hegseth has also threatened to reduce Washington's NATO spending unless allies reached defense spending targets.

Washington has been clear with Europe that it wants NATO allies on the continent to take over primary responsibility for their own conventional defense as US focus shifts towards China.

As part of that process the Pentagon has already told allies it is reducing the number of assets worldwide that it makes available for NATO operations.