U.S. President Donald Trump will meet other NATO leaders in Brussels on May 25, the Belgian government said on Thursday, although the alliance said the date was still undecided.

Prime Minister Charles Michel said on Twitter late on Wednesday: "Very pleased to welcome my colleagues for the next NATO summit in Brussels on 25th May and opening of new headquarters."

New U.S. presidents typically come to NATO but Trump's description of the North Atlantic Treaty Organisation as "obsolete" during his election campaign has made the summit all the more important to cement transatlantic ties, diplomats say.

NATO and the U.S. mission to the alliance said they could not yet confirm the date. "Consultations on the exact date are ongoing among allies," a NATO official said. Trump and NATO Secretary-General Jens Stoltenberg have agreed to hold a summit in late May.

Belgian sources said the government, which arranges security in the capital for the event, had been assured that the date was fixed.

The summit is due to be held in the new NATO headquarters in Brussels, which has been under construction since 2010 and will replace the current HQ, built in 1967.