The British declaration adds further pressure on Eastern flank countries to drop their hesitation over Rutte, criticized for failing to bring Dutch defense spending up to the 2 percent of GDP target pledged by all NATO allies.

Still, Rutte is the clear favorite among big allies.

He emerged Wednesday as a frontrunner in what is so far a one-man race for the alliance’s top job, expected to be vacated by Stoltenberg in October.

Rutte has already secured the backing of two-thirds of NATO countries to lead the military alliance, two senior officials told POLITICO Wednesday.

Later in the day, a U.S. official confirmed that Biden is one of those supporting Rutte, which is likely to sway more allies to get on board with Rutte’s nomination.

Rutte needs the backing of all 31 NATO members before he can be declared the secretary-general.