Americans on Sunday remembered the 21th anniversary of the September 11 attacks, with US President Joe Biden vowing to continue the fight against terrorism.

The president, who is preparing for crucial midterm elections in less than two months, also used his 9/11 speech as an opportunity to take jabs at his Republican predecessor, Donald Trump.

In a speech delivered from the Pentagon, one of the targets during the attack which killed nearly 3,000 people, Biden urged Americans to stand up for "the very democracy that guarantees the right to freedom that those terrorists on 9/11 sought to bury in the burning fire, smoke and ash."

On September 11, 2001, militant group al-Qaeda carried out what was later known as the biggest terrorist attack on US soil. Al-Qaeda terrorists hijacked four jets and flew three into the World Trade Center towers in New York and the Pentagon in Arlington, Virginia.

The fourth plane crashed near Shanksville, Pennsylvania after its passengers overcame the hijacker.

The attack triggered the US's yearslong "War on Terror," which aims to confront global terrorism.  It involved a two-decade-long war on Afghanistan to root out al-Qaeda, which ended with a full US withdrawal last year that saw the fundamentalist Taliban retake Kabul.

Biden: 'We'll never forget. We'll never give up.' 

Despite continuing criticism of the US's withdrawal from Afghanistan, Biden stressed that the "War on Terror" was far from over.

"We will not rest. We'll never forget. We'll never give up," he said on Sunday.

Just about a month ago, the US president celebrated the killing of al-Qaeda leader Ayman al-Zawahiri in a drone strike in Kabul. The Egyptian-born surgeon turned militant was at the top of the FBI's most-wanted list since the killing of Al-Qaeda founder Osama Bin Laden 11 years ago.

Elsewhere in the US, ceremonies were held at other locations targeted by the militant group, remembering and honoring the victims. First Lady Jill Biden witnessed a ceremony held in Pennsylvania, whereas Vice President Kamala Harris attended another memorial event in New York City.

Al-Qaeda releases book on 9/11 timeline, aims 

Meanwhile, the al-Qaeda terrorist network released a book unveiling the timeline ahead of the attacks. The book, which was published online by al-Qaeda's media wing As-Sahab, also detailed the group's aims behind the terrorist acts.

Written by a senior member of the group, the nearly 250-page book said al-Qaeda had planned to target US interests to drag the country into a long-term war of "attrition." 

The writer of the book, Abu Muhammad al-Masri, is believed to have been killed in Iran in 2020.

rmt/wd (AFP, AP, dpa, Reuters)