The Trump administration has trumpeted a breakthrough in talks with Russia this month, claiming that Mr. Putin had accepted a proposal for the West to provide security guarantees for Ukraine as strong as Article 5 of the NATO charter, which stipulates that an attack on one alliance member is considered an attack on all.

Foreign Minister Sergei Lavrov said any Western security guarantees for Ukraine in an eventual peace deal could only be enacted with Moscow’s cooperation—essentially rendering them useless against another Russian invasion.

Ukraine and its European allies have been buoyed by Trump's promise during a summit on Monday of security guarantees for Kyiv, but many questions remain unanswered. Officials told Reuters that the Pentagon is carrying out planning exercises on the support Washington could offer beyond providing weapons.