BRUSSELS — One of NATO’s longest serving top officials, Secretary-General Jens Stoltenberg, stepped down on Tuesday, handing over the reins to former Dutch Prime Minister Mark Rutte as the military alliance confronts some of the biggest challenges in its history.

General Mark Rutte has visited Ukraine in his first official trip since taking office and pledging continued support for Kyiv.

President Volodymyr Zelenskiy will meet NATO General Secretary Mark Rutte and British Prime Minister Keir Starmer in London on Thursday, a boost for Ukraine after a summit of its main backers was cancelled at a difficult moment in its fight against Russia.