Nation grieves for 9/11 victims: Trump

President Donald Trump has told a September 11 memorial service at the Pentagon the nation grieves for the people "who were murdered by terrorists" 16 years ago.

The president and first lady Melania Trump joined Defense Secretary Jim Mattis, members of cabinet and military personnel at the Pentagon to observe the anniversary of the attacks on the nation's defence headquarters on Monday.

The president warned extremists, saying "America cannot be intimidated" and those who try will join the list of enemies "who dared to test our mettle".

When America is united "no force on earth can break us apart".

The native New Yorker is observing the September 11 anniversary for the first time as president. Nearly 3000 people were killed when four hijacked planes hit the World Trade Center, the Pentagon and at a Pennsylvania field.

Earlier on Tuesday, moments of silence were held to observe when the south World Trade Center tower collapsed and when United Airlines Flight 93 crashed near Shanksville, Pennsylvania.

Five bells also tolled to mark the moments the World Trade Towers were struck by planes and fell, as well as the Pentagon attack and the United flight where passengers took on their hijackers and crashed another plane.