"In the current situation, the French and British nuclear forces are a complement to U.S. extended deterrence, but they would not constitute a viable solution in the event of an abrupt withdrawal of U.S. nuclear forces," the Washington-based Centre for Strategic & International Studies said.

The breakdown in U.S.-Ukraine ties has given new urgency to the two European nuclear powers' efforts to pull together ideas for a peace plan that would initially outline a short truce but also eventually include broader security guarantees. Britain and France have both said the U.S. would be needed for future assurances.

To face that threat without the United States, Macron said he had decided to open a strategic debate on expanding France's nuclear deterrent to protect the country's European allies — something incoming German Chancellor Friedrich Merz has also advocated.

The French defense minister says France is also seeking access to Ukraine’s deposits of critical minerals, indicating that the United States isn’t the only player.