NATO members are expected to agree a major increase in defence spending under pressure from US President Donald Trump at the gathering.

"The decision will be made on the eve of the summit. This is just the schedule," the Ukrainian source said, describing the meeting as "an opportunity to maintain support and promote a ceasefire".

Diplomats and officials at NATO say Zelensky is invited to a formal dinner for leaders hosted by the Dutch king on the evening of 24 June, which Trump is expected to attend.

Zelensky will also likely attend a forum on the defence industry being held on the sidelines of the summit.

But there will be no formal working session between the Ukrainian president and NATO's 32 leaders as there was at the previous NATO summit in Washington.

Zelensky wants NATO to offer security guarantees to Ukraine in the event of a ceasefire or peace deal with Russia -- something Moscow has called "unacceptable".