Trump repeated demands on the second day of the summit for countries to meet a spending target of two percent of GDP now, instead of by 2024, and to eventually double spending to four percent.

During a meeting of the allies with Ukraine and Georgia, "Trump took advantage of his speaking time to return to the issue of burden sharing", one of the diplomatic sources said on condition of anonymity.

NATO chief Jens Stoltenberg "has therefore convened a meeting in Atlantic Council format, which involves all 29 countries in the alliance."

The source refused to comment on reports that Trump had threatened to pull out of NATO altogether.

Another diplomatic source said that a "special session on burden sharing is ongoing."

Trump also singled out Germany and its chancellor Angela Merkel for criticism, a day after saying Berlin had become a "captive of Russia" because of a gas pipeline deal and should be paying more for defence.

"Trump has pointed to several countries, especially Germany," the first source said, adding that there was "hard language, with repeated references to Germany" and even to the German chancellor, "calling her Angela".