President Barack Obama on Saturday said that whatever the circumstances, European allies in NATO would always be able to rely on the United States.

"In good times, and in bad, Europe can count on the United States. Always," Obama said at the close of a two-day NATO summit in Warsaw which approved the alliance's biggest military upgrade since the end of the Cold War.

The president said the 28 nation alliance was at a "pivotal moment... In the nearly 70 years of NATO perhaps never have we faced such a range of challenges all at once, security, humanitarian, political."

He cited first the Islamic State group threat, then Russia's intervention in Ukraine, Europe's worst migrant crisis since the end of World War II and finally Britain's vote to quit the European Union which has stoked growing fears about the continent's future.

"In this challenging moment, I want to take the opportunity to state clearly what will never change. And that is the unwavering commitment of the United States to the security and defense of Europe," he said.

But at the same time, Obama warned that the NATO allies had to do their part too, especially meeting a commitment to reverse years of cuts and devote two percent of annual economic output to defence spending.

Britain, Poland, Greece and Estonia were on target but "that means that the majority of allies are still not hitting that two percent mark," Obama said.

"So we had a very candid conversation about this," he added.