The United States commemorated the victims of the September 11, 2001, terrorist attacks with a minute's silence on Sunday, 15 years to the day after extremists flew hijacked planes into New York's World Trade Center and the Pentagon.

In a sombre commemoration, which has almost become a tradition, families of victims gathered in New York City and elsewhere in the United States to mark the anniversary.

The minute's silence began at 8:46 am (1246 GMT), which marks the time the first hijacked airplane hit the North Tower of the World Trade Center.

The names of the 2,977 victims were due to be read during the main memorial service at the National September 11 Memorial and Museum in New York.

More than 23 million people have visited the memorial site since it opened in September 2011, and more than 4 million visitors have been to the museum, which opened in May 2014.

Terrorists on suicide missions carried out the 2001 attacks, hijacking four passenger jets and crashing them into the two World Trade Center towers in New York and the Pentagon defence headquarters outside Washington.

Services were also scheduled for Washington and a memorial site just outside of Shanksville, Pennsylvania, where the fourth plane crashed after its passengers apparently rose up against the hijackers and crashed it in a field, thwarting an attack on Washington.

President Barack Obama and First Lady Michelle Obama observed the minute's silence in the White House.

The 15th anniversary of the deadliest terrorist attack on US soil will provide a moment to reflect on what has changed since the September morning in 2001 that has shaped US domestic and foreign policy since.

In the 15 years since the attack, nearly every minute, every detail, every anecdote from victims, witnesses and relatives have been reconstructed in documentaries, feature films, fiction and non-fiction books.

There are plays, TV series and poems about 9/11, and it has been referenced in rock ballads, rap lyrics and classical music compositions. A painting by the artist Gerhard Richter called "September" depicts the silhouettes of the collapsed twin towers.

The fear of terrorism has remained vivid also as the issue has played a prominent role in the 2016 US presidential elections.

The US public's fear of such attacks has reached new highs in 2016, according to a recent survey by the Pew Research Center.

Currently, 40 per cent of the US public believes that the ability of terrorists to launch another major attack against the US is greater than it was at the time of the 2001 attacks, which is the largest proportion since 2002.

The shift is especially noticeable when divided along party lines - 58 per cent of Republican respondents said that terrorists' abilities to launch an attack have increased since 2001. The figure stands at 31 per cent for Democrats and 34 per cent for independent voters.

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