White House spokeswoman Karine Jean-Pierre said the pair would review preparations for the summit in Lithuania and "discuss Allies' support for Ukraine in the face of Russia's brutal war of aggression."

NATO leaders are due to convene on July 11-12 in the capital Vilnius.

Stoltenberg's mandate as NATO chief is due to end in October. When asked about a possible extension of that term, Jean-Pierre did not want to comment, but said: "The secretary has done an outstanding job... in this critical moment in history."

The White House is "very, very grateful for his leadership," she told reporters.