United States President Donald Trump on Monday paid tribute to the 2,977 people killed on September 11, 2001, warning that “savage killers” who threaten US will find no haven on earth.

On the 16th anniversary of the attacks, Trump's first as president, he observed a moment of silence at the White House before laying a wreath and delivering remarks at the Pentagon, where 184 people died.

His tone unbending and solemn, Trump championed America's resilience and “common bonds,” but issued a stern warning to “enemies” that “America cannot be intimidated.”

“Those who try will soon join the long list of vanquished enemies who dared to test our mettle,” he said, as a drone of jet engines carried echoes of a day half a generation ago that many Americans have vowed not to forget.

In Trump's native New York, at Ground Zero, there was a minute's silence at 8:46 am, the moment the first of two hijacked airliners struck the World Trade Centre.

In all, four planes were hijacked by Al-Qaeda militants who used them to topple the trade centre's twin towers and hit the Pentagon.

The fourth plane, Flight 93, crashed in a field in Shanksville, Pennsylvania, where Vice President Mike Pence travelled for the day.

“It was the worst attack on our country since Pearl Harbour and even worse because this was an attack on civilians — innocent men, women and children whose lives were taken so needlessly,” Trump said.

The attacks remain the deadliest ever on US soil, plunging the United States into a chain of rolling wars against militants, in which Trump has vowed to give no quarter.

Next year, Americans who were born after 9/11 are due to be deployed to Afghanistan and Iraq for the first time.

“We're ensuring that they never again have a safe haven to launch attacks against our country,” Trump said.

“We are making plain to these savage killers that there is no dark corner beyond our reach, no sanctuary beyond our grasp and nowhere to hide anywhere on this very large earth.”