President Trump delivered a stern warning to extremist groups threatening the U.S. on the anniversary of the Sept. 11, 2001, terror attacks, his first commemoration of the somber occasion as president.

Speaking at the Pentagon on Monday, where one of four hijacked planes crashed 16 years ago, Trump mourned those who died and said he would honor the sacrifice by doing “whatever we must to keep our people safe.”

“The terrorists who attacked us thought they could incite fear and weaken our spirit,” he said. “But America cannot be intimidated, and those who try will soon join the list of vanquished enemies who dared to test our mettle.” 

He vowed U.S. armed forces would ensure al Qaeda and other extremist groups would never be given a “sanctuary” from where they could plot and launch a similar attack again.