NATO will host a summit of leaders including US President Donald Trump in May at its new Brussels headquarters, even though it will not be ready by then, officials and sources said Thursday.

Security and IT systems at the $1.2-billion headquarters in the Belgian capital are behind schedule and so the transatlantic military alliance will only properly move in come September, sources told AFP.

Belgian Prime Minister Charles Michel said on Twitter late Wednesday that the meeting of 28 leaders would take place on May 25 and that the building would be opened then.

"Very pleased to welcome my colleagues for the next NATO summit in Brussels on 25th May and opening of new headquarters," Michel tweeted.

But NATO would not confirm the date or the venue, saying only that Secretary General Jens Stoltenberg and Trump had agreed by phone on Sunday that the summit would be in late May.

"Consultations on the exact date are ongoing among allies. As to the summit venue, we aim to hold it at the new Headquarters," a NATO official told AFP.

Trump has previously criticised NATO, calling it "obsolete" in an interview earlier this year and pressing the rest of the 28-nation group to commit more money to it.

A source close to the matter told AFP that NATO aimed to inaugurate its new building after the summer holidays, "in my view more like the end of September."

- 'Slightly behind schedule' -

NATO spokeswoman Oana Lungescu said earlier that the move would be a "complex logistical undertaking" involving more than 4,000 NATO staff and delegations from the 28 allies.

"The move to the new NATO headquarters has started and it's due to be completed later this year," she said in a statement to AFP.

"The new IT and security systems in the building are highly complex and we are slightly behind schedule in making all of them fully operational."

The supplier responsible for delivering the network infrastructure has still not delivered it to the alliance, which has in turn pushed back security tests, the source close to the matter said.

The new NATO HQ, with its distinctive shape of two double lightning bolts, is built just over the road from the current 1960s-era headquarters near Brussels airport.

It had been scheduled to open in the first quarter of 2017.

The White House confirmed last week that Trump will attend the May summit, easing doubts in Europe about the new US president's commitment to the bloc.

Trump expressed "strong support for NATO" in a phone call with Stoltenberg but also urged European members to pitch in more to ease the defence spending burden.

Trump's apparent coolness towards the alliance has been particularly alarming for some member nations given his friendly stance towards Russia, which NATO has described as being increasingly assertive in the wake of its seizure of Crimea from Ukraine in 2014.