Americans are commemorating September 11 with mournful ceremonies, volunteering, appeals to “never forget” and rising attention to the terror attacks’ extended toll on responders.

A crowd of victims’ relatives was expected at ground zero on Wednesday, while President Donald Trump was scheduled to join an observance at the Pentagon.

Vice President Mike Pence was to speak at the third attack site, near Shanksville, Pennsylvania.

Eighteen years after the deadliest terrorist attack on American soil, the nation is still grappling with the aftermath at ground zero, in Congress and beyond.

A rocket exploded at the US embassy as the anniversary began in Afghanistan, where a post-September 11 invasion has become America’s longest war.

“People say, ‘Why do you stand here, year after year?’,” Chundera Epps, a sister of victim Christopher Epps, said at last year’s ceremony at the World Trade Centre.

“Because soldiers are still dying for our freedom. First responders are still dying and being ill.”

“We can’t forget. Life won’t let us forget,” she added.

The anniversary ceremonies centre on remembering the nearly 3000 people killed when hijacked planes rammed into the World Trade Centre, the Pentagon and a field near Shanksville in 2001.

Friends and family gathered at a memorial service for New York City firefighter Michael Haub in Franklin Square on Tuesday. Last week, the New York City medical examiner identified more of his remains recovered at ground zero. https://t.co/GZs5WL87Pa

— Stars and Stripes (@starsandstripes) September 11, 2019

The victims’ names are read aloud at the ground zero ceremony, where moments of silence and tolling bells mark the moments when the aircraft crashed and the centre’s twin towers fell.

But there has been growing awareness in recent years of the suffering of another group of people tied to the tragedy: fire fighters, police and others who died or fell ill after exposure to the wreckage and the toxins unleashed in it.

The sick gained new recognition this year at the memorial plaza at ground zero, where the new 9/11 Memorial Glade was dedicated this spring.

The tribute features six large stacks of granite inlaid with salvaged trade centre steel, with a dedication “to those whose actions in our time of need led to their injury, sickness, and death.” No one is named specifically.

Some memorials elsewhere already included sickened rescue, recovery and clean-up workers, but those who fell ill or were injured, and their families, say having a tribute at ground zero carries special significance.

September 11 is also known as a day of service, with people across the US volunteering at a range of charitable endeavours on and near the anniversary.

-AAP