Washington, Sep 12 (IANS): US President Donald Trump and First Lady Melania Trump have attended an observance in Pennsylvania -- the site of where a fourth plane crash had crashed on September 11, 2001.

On the 17th anniversary of the attacks the Trumps attended the ceremony in Shanksville "where a new Tower of Voices memorialises the victims of Flight 93, which crashed there 17 years ago, the White House said.

Trump marked the anniversary by paying tribute to his lawyer and former New York City Mayor Rudy Giuliani before heading to the memorial service

"Rudy Giuliani did a great job as Mayor of New York City during the period ... His leadership, bravery and skill must never be forgotten. Rudy is a TRUE WARRIOR!" Trump tweeted on Tuesday.

The First Lady took to Twitter with an image from the moment of silence held last year at the White House. "#NeverForget 9.11.01," she tweeted.

US Press Secretary Sarah Sanders tweeted:"We received a terrible reminder of the evil in our world. In the midst of horrific tragedy America stood strong.

"We will never be the same, but we will always resilient. Today we honour the memory of those who died, and those who keep us safe."

On September 11, 2001, 19 militants associated with the Al Qaeda hijacked four airplanes and carried out suicide attacks against targets in the US.

Two of the planes were flown into the twin towers of the World Trade Centre in New York City, a third plane hit the Pentagon just outside Washington, D.C., and the fourth plane crashed in a field in Pennsylvania.

Almost 3,000 people were killed during the 9/11 terrorist attacks, which triggered major US initiatives to combat terrorism and defined the presidency of George W. Bush.