US President Donald Trump has agreed to attend the upcoming NATO summit slated to be held in May, after having repeatedly criticised the alliance and having called it "obsolete", the media reported.

Trump spoke with the North Atlantic Treaty Organisation Secretary General Jens Stoltenberg on Sunday evening regarding the US' "strong support for NATO", the White House press office announced in a statement.

According to the statement, the two leaders discussed "how to encourage all NATO allies to meet their defence spending commitments" and the potential for a peaceful resolution of the conflict along the Ukrainian border, NBC News reported.

During his campaign, Trump set off alarms in Europe after suggesting that he might set conditions for defending members of the alliance under attack.

Last July, Trump told The New York Times that the US was shouldering too much of the cost for the security alliance.

Trump has also repeatedly called NATO obsolete, most recently in an interview with Germany's Bild daily in January.  

Also in January, Stoltenberg said he had a phone call with Trump after he won the November 8 presidential election and was sure he would remain strongly committed to the institution.

"I am absolutely certain that the new president and the new administration will be strongly committed to a strong NATO," Stoltenberg told CNBC last month at the World Economic Forum in Davos, Switzerland.

British Prime Minister Theresa May, who sought reassurances about Trump's commitment to NATO during her visit to the White House also said Trump told her the United States is "100 per cent behind NATO".