US Secretary of State John Kerry reaffirmed Washington's promise to protect its NATO allies Thursday after White House hopeful Donald Trump called it into question.

Kerry would not be drawn on Trump's claim that he would evaluate an ally's request for assistance based on how well he judged it to be paying its way in the alliance.

But he took the opportunity of a news conference to mark the end of a summit between members of the coalition against the Islamic State group to restate US policy.

"I'm not going to get into the presidential race," he said. "I'm not in politics.

"So let me just restate American policy with respect to NATO, because I want our NATO partners to be clear about where we stand," he continued.

"This administration, like every single administration Republican and Democrat alike since 1949 remains fully committed to the NATO alliance and to our security commitments under Article Five, which is absolutely bedrock, to our membership and partnership with NATO."

Under Article Five of the NATO treaty, members agree to regard an attack on one member as an attack on all and promise to mobilize together to defeat the threat.

In an interview published Thursday in the New York Times, Trump called this vow into question.

He warned that he would only come to the aid of the Baltic states in the event they were invaded by Russia if he judged they had "fulfilled their obligations to us."

This drew a furious response from Estonia and Latvia, and outrage from Trump's domestic political critics.