Two U.S. officials who spoke on the condition of anonymity said the arrangement would be similar to the NATO mutual defense agreement. But crucial details remain unsettled as negotiators seek a deal that Russia will approve.

A 1,000-strong "dark fleet" of rogue tankers evading sanctions has become a new target for the U.S. and Ukraine, according to a senior maritime intelligence analyst.

European leaders said they had agreed on Monday any decisions on potential Ukrainian territorial concessions to Russia could only be made once robust security guarantees were in place which should include a European-led multinational force.

Ukraine has relinquished its ambition of joining the NATO military alliance in exchange for Western security guarantees as a compromise to end the war with Russia, President Volodymyr Zelenskyy said ahead of talks with U.S. envoys in Berlin.

Germany plans to deepen its defense cooperation with Ukraine, including for the first time the possibility of acquiring weapons and battlefield data from Kyiv, alongside a potential peace agreement under discussion in Berlin.

U.S. peace negotiators have told Ukraine during peace talks in Berlin that it must agree to withdraw its forces from the eastern Donetsk region as part of any deal to end the nearly four-year-old war,