Jens Stoltenberg is expected to be asked to remain as NATO secretary-general for another year, according to a source familiar with the discussions and a U.S. official, after the alliance has struggled to decide on a replacement ahead of a mid-July summit in Lithuania.

Stoltenberg, a Norwegian who has spent nine years in the post and is due to step down at the end of September, has broad support among the alliance and continues to be an effective leader, the source, who requested anonymity, said.

"The (Biden) administration is coming around the idea of Stoltenberg staying on for another year," said the U.S. official, who also spoke on condition of anonymity.