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NATO Secretary-General Jens Stoltenberg is playing down differences among member countries as the military alliance marks its 70th anniversary in Washington this week.

Stoltenberg concedes there are spats over trade, climate change and the Iran nuclear deal but he insists the 29 allies are united on the need to defend each other, particularly faced with an assertive Russia.

He said Monday that "the strength of NATO is that despite these differences, we have always been able to unite around our core tasks. That is, to protect and defend each other."

Stoltenberg meets Tuesday with U.S. President Donald Trump and will address Congress before foreign ministers on Thursday mark the anniversary of the signing of NATO's founding Washington Treaty.