NATO's secretary general, responding to Donald Trump's criticism of the alliance, recalled Wednesday how it stood behind America after the September 11, 2001 terror attacks.

The first and only time NATO invoked "collective defense goals" was after the suicide plane bombings against New York and Washington, NATO chief Jens Stoltenberg said in a speech in Washington.

"Europeans stepped up to help and support our ally, the United states," he added.

NATO then quickly joined the United States in invading Afghanistan on the grounds it was harboring al-Qaeda, blamed for 9/11. And more than 1,000 European and Canadian soldiers lost their lives there, the secretary general said.

Trump, the Republican presidential frontrunner, recently derided NATO as obsolete and said the United States covers an oversized portion of its budget.

President Barack Obama said Monday that NATO remains a cornerstone of US defense policy.

Stoltenberg said European countries are aware they need to boost defense spending and rely less on the United States to defend them.