The outcome of their call is expected to influence the president’s discussions with his Ukrainian counterpart, Volodymyr Zelensky, on Friday over the prospects of the U.S. providing a powerful new weapon system.

A future visit by Russian ruler Vladimir Putin to Hungary, a member of both the European Union and NATO, will put Ukraine's allies in an awkward position. Details: The choice of a country belonging to alliances leading international efforts to help Ukraine and isolate Russia for hosting the summit has surprised diplomats and analysts almost as much as the meeting itself.