The Queen has sent her respects to US President Joe Biden on the 20th anniversary of 9/11.

In a statement, she recalled the "terrible" attacks 20 years ago, and spoke of the loss of people from "many nations, faiths and backgrounds".

The Queen said her thoughts and prayers were with all those who suffered loss on 11 September 2001, but she also paid tribute to the "resilience" of communities that rebuilt after the attacks.

"My thoughts and prayers — and those of my family and the entire nation — remain with the victims, survivors and families affected, as well as the first responders and rescue workers called to duty," the Queen said in her message to Biden.

"My visit to the site of the World Trade Center in 2010 is held fast in my memory.

"It reminds me that as we honour those from many nations, faiths and backgrounds who lost their lives, we also pay tribute to the resilience and determination of the communities who joined together to rebuild."

A total of 2,977 people died when hijacked planes were flown into the Twin Towers and the Pentagon, and crashed into a field in Pennsylvania. 

Commemorative events are taking place in the US on Saturday, as well as in the UK to remember the 67 British victims of the tragedy.

Biden will attend a ceremony at the 9/11 Memorial at the site where the Twin Towers once stood in New York City. 

Speaking on the eve of the anniversary, he said the attackers could not destroy the unity of the US.

"We honour all those who risked and gave their lives in the minutes, hours, months and years afterwards," he said. 

"No matter how much time has passed, these commemorations bring everything painfully back as if you just got the news a few seconds ago."

He added, "We learned that unity is the one thing that must never break." 

British Prime Minister Boris Johnson said the world was coming together on the anniversary "in sorrow but also in faith and resolve." 

He said the recent events in Afghanistan would "only strengthen our determination to remember those who were taken from us, cherish the survivors and those who still grieve and hold fast to our belief in liberty and democracy, which will always prevail over every foe."

"Twenty years ago, September 11 2001 became, in President Roosevelt's words after Pearl Harbour, a 'date which will live in infamy'," he said.

"On a crystal clear morning, terrorists attacked the United States with the simple goal of killing or maiming as many human beings as possible, and by inflicting such bloodshed in the world's greatest democracy, they tried to destroy the faith of free peoples everywhere in the open societies which terrorists despise and which we cherish.

"And it is precisely because of the openness and tolerance of the United States that people of almost every nationality and religion were among the 2,977 murdered on that day, including 67 Britons, each of them a symbol of the eternal friendship between the United Kingdom and the United States.

"But while the terrorists imposed their burden of grief and suffering, and while the threat persists today, we can now say with the perspective of 20 years that they failed to shake our belief in freedom and democracy; they failed to drive our nations apart, or cause us to abandon our values, or to live in permanent fear.

"The fact that we are coming together today – in sorrow but also in faith and resolve – demonstrates the failure of terrorism and the strength of the bonds between us."