In New York, a dispute over coronavirus-safety precautions is leading to split-screen remembrances on Friday, one at the September 11 memorial plaza at the World Trade Center and another on a nearby corner.

The Pentagon's observance will be so restricted that not even victims' families can attend, though small groups can visit the memorial there later in the day.

Trump and Biden are both headed - at different times - to the Flight 93 National Memorial near Shanksville, Pennsylvania.

President Trump is speaking at the morning ceremony, the White House said. Democratic challenger Biden plans to pay respects there in the afternoon after attending the observance at the 9/11 memorial in New York.

Meanwhile, Vice President Mike Pence is also due at ground zero - and then at the alternate ceremony a few blocks away.

The families of 9/11 victims say it's important for the nation to pause and remember the hijacked-plane attacks that killed nearly 3000 people at the trade centre, at the Pentagon and near Shanksville on September 11, 2001.

Though the candidates will be focused on the commemorations, the political significance of their focus on Shanksville is hard to ignore: Pennsylvania is a must-win state for both. Trump won it by less than a percentage point in 2016.

Around the country, some communities have cancelled 9/11 commemorations because of the pandemic, while others are going ahead, sometimes with modifications.

The New York memorial is changing one of its ceremony's central traditions: having relatives read the names of the dead, often adding poignant tributes.

Thousands of family members are still invited. But they'll hear a recording of the names from speakers spread around the vast plaza, a plan that memorial leaders felt would avoid close contact at a stage but still allow families to remember their loved ones at the place where they died.

But some victims' relatives felt the change robbed the observance of its emotional impact.

A different 9/11-related group, the Stephen Siller Tunnel to Towers Foundation, set up its own, simultaneous ceremony a few blocks away, saying there's no reason that people can't recite names while keeping a safe distance.

The two organisations also tussled over the Tribute in Light, a pair of powerful beams that shine into the night sky near the trade centre and evoke its fallen twin towers.

The 9/11 memorial initially canceled the display, citing virus-safety concerns for the installation crew. After the Tunnel to Towers Foundation vowed to put up the lights instead, the memorial changed course with help from its chairman, former Mayor Mike Bloomberg, and Governor Andrew Cuomo.

Australian Associated Press