NEW YORK - Americans commemorated the 15th anniversary of the September 11, 2001, terrorist attacks on the United States that left nearly 3,000 people dead.

Ceremonies were held on Sunday across the country to remember the victims, including the recital of the names of the dead, tolling church bells and a tribute at the site where New York City's twin towers tumbled.

The names of the victims were read slowly by relatives during a ceremony at the 9/11 Memorial plaza in Lower Manhattan.

In Washington, President Barack Obama said the attack had been one of the darkest days in the history of the country.

The 9/11 attacks killed 2,983 people and caused about $10 billion worth of property and infrastructure damage.

Presidential hopefuls Hillary Clinton and Donald Trump were in the crowd in New York City, but public officials do not speak on this occasion in keeping with the tradition that began in 2012.

"We'll never forget the horror of Sept 11, 2001," Clinton said in a brief statement issued to the press. "Let's honour the lives and tremendous spirit of the victims and responders."

Trump said in a statement that it was a day of sadness and remembrance, but also of resolve. But Clinton left the ceremony early because she didn't feel well, her campaign said Sunday.

"During the ceremony, she felt overheated so departed to go to her daughter's apartment, and is feeling much better," said Clinton spokesman Nick Merrill.

He said Clinton "attended the September 11th Commemoration Ceremony for just an hour and thirty minutes this morning to pay her respects and greet some of the families of the fallen."

More than 340 firefighters and 60 police were killed on that Tuesday morning in 2001. Many of the first responders died while running up stairs in the hope of reaching victims trapped on the towers' higher floors.

Houses of worship throughout the city tolled their bells at 8:46 am EDT, the time American Airlines Flight 11 slammed into the North Tower.

Memorial ceremonies also are underway at the sites of the other crash sites. President Obama participated in a wreath ceremony at the Pentagon, paying tribute to the victims. "No deed we do can ever truly erase the pain of their absence," Obama said. "Your steadfast love and faithfulness has been an inspiration to me and our entire country."

In New York City, crowds for the ceremony have diminished over the years. But this year, with a significant anniversary falling on a weekend, more people attended.

This year's 9/11 memorial comes just days after the US Congress passed a controversial legislation that allows relatives of the victims to sue Saudi Arabia for compensation.

Saudi Arabia has strongly opposed the legislation. It has threatened to sell off $750 billion in American assets if it becomes law.

AFP adds: The 9/11 was the first foreign attack on the US mainland in almost two centuries and sparked US-led invasions of Afghanistan (2001) and Iraq (2003), where war rages on more than a decade later.

Countries in the Middle East from Libya to Syria are engulfed in war, providing a fertile ground for Al-Qaeda affiliates to breed, and Europe has been tormented by attacks inspired by the Islamic State group.

Held at the September 11 memorial, the service paused six times - to mark the moments when each of the two planes hit, when each tower fell.

George W Bush, who was president at the time of the attacks, is not expected to attend either ceremony in New York or Washington.

Instead, his office said he will go to church in Dallas, Texas and then attend the Dallas Cowboys home opener against the New York Giants, where he will take part in the ceremonial coin toss with two New York police officers who were at Ground Zero on 9/11.

Sunday marks the start of the NFL season in the United States, and those attending the American football games and watching on television will watch video messages from both Obama and Bush.