The United States is remembering the nearly 3000 people killed in the September 11, 2001 attacks on New York and Washington DC with smaller events this year, part of efforts to enforce distancing during the ongoing pandemic.

US President Donald Trump marked the 19th anniversary on Friday in Pennsylvania at the site where one of the four hijacked planes crashed after passengers attempted to regain control from the hijackers.

After the names of the 40 deceased passengers and crew were recited, Mr Trump urged unity.

“Our sacred task, our righteous duty and our solemn pledge is to carry forward the noble legacy of the brave souls who gave their lives for us 19 years ago,” he said.

“In their memory, we resolve to stand united as one American nation, to defend our freedoms, to uphold our values, to love our neighbours, to cherish our country, to care for our communities, to honour our heroes, and to never, ever forget.”

President Donald J. Trump, First Lady Melania Trump and Interior Secretary David Bernhardt pledge allegiance to the flag during a ceremony at the Flight 93 National Memorial Pennsylvania. Photo: Getty

Joe Biden, the Democratic presidential nominee, attended a morning ceremony at the 9/11 memorial in New York where attackers flew two aeroplanes into the Twin Towers of the World Trade Center.

Vice President Mike Pence and New York Governor Andrew Cuomo also attended the commemoration, at which attendees were required to wear masks and stay physically distant.

Mr Biden and Mr Pence exchanged elbow bumps, a popular greeting in the coronavirus era, displaying unity amid campaigning season and a time of stark political division.

Democrat presidential candidate Joe Biden and US vice president Mike Pence do a COVID greeting on the anniversary of 9/11 in New York. Photo: Getty

Mr Biden will later also travel to Pennsylvania, a key battleground state in the November presidential election, though he is not expected to cross paths with Trump.

Beams of light shone into the sky over New York overnight from where the Twin Towers once stood.

A similar beam came up from the Pentagon just outside Washington DC, which was also hit by the attackers.

Flags are at half-staff, including at the White House, and bells tolled in New York City to mark the deaths.

Rose tributes to the victims of 9/11 in New York. Photo: Getty

Many of the normal memorial events were scaled back as a safety precaution, with fewer speeches and some aspects limited to immediate families only.

In New York City, the traditional reading of victims’ names by relatives was called off at the 9/11 memorial, with pre-recorded playbacks broadcast instead.

Unhappy with the changes, a foundation conducted a simultaneous ceremony nearby in Manhattan, at which family members recited the names of their loved ones while keeping at a distance.

Mr Pence attended both events.