Days after the killing of Osama bin Laden, U.S. President Barack Obama met New York fire-fighters and police on Thursday and visited Ground Zero to offer comfort to a city still scarred by the September 11 attacks.

His predecessor George W. Bush, just three days after hijacked planes destroyed the World Trade Centre’s Twin Towers, stood bullhorn in hand in the smouldering wreckage to declare: "The people who knocked these buildings down will hear all of us soon."

Almost a decade later, in a bookend to that historic visit, Obama came to New York to say that promise had been kept.

He said the killing of bin Laden told the world, "that when we say we will never forget, we mean what we say."

Obama visited the 'Pride of Manhattan' Engine 54 firehouse in midtown, which lost 15 members in the attacks, before heading to Lower Manhattan to talk with police and lay a wreath at Ground Zero where he met with victims' families.

Obama shook hands with fire-fighters and told them, "I wanted to just come here to thank you."

"This is a symbolic site of the extraordinary sacrifice that was made on that terrible day almost 10 years ago."

"It didn't matter who was in charge, we were going to make sure that the perpetrators of that horrible act - that they received justice," Obama said.

Bin Laden, the al Qaeda leader who masterminded the September 11, 2001, attacks, was shot in the head by U.S. forces who stormed his compound in Pakistan on Monday after a decade-long manhunt. Nearly 3,000 people were killed when al Qaeda hijackers crashed commercial planes into the Twin Towers, the Pentagon outside Washington, D.C., and a Pennsylvania field.

"It's a good thing he is coming to visit," said Al Fiammetta, 57, a safety engineer from Bellport, New York, who said he worked at Ground Zero clearing debris and waited to see Obama. "We have been waiting for this for 10 years. It puts a little more American pride in people."

New York City resident Caroline Epner, 32 and seven months pregnant, said, "It's OK for him to take a victory lap."

Red, white and blue

Later Obama met with New York police and thanked them for doing extraordinary acts without fanfare and urged them to remain vigilant, saying there remain threats from extremists.

At Ground Zero during a bright and sunny afternoon, Obama laid a wreath of red, white and blue flowers to honour those who died. Obama then paused, bowed his head, closed his eyes and held his hands together for a moment of silence.

Obama, who made no remarks at the site, greeted relatives of victims. The brief ceremony took place by the "Survivor's Tree," which amazingly survived the attacks and was nursed back to health and then returned to be part of the memorial that will open here on the 10th anniversary of the attacks.

He stood in a place that almost a decade ago was the pulverized remains of what were once the world's tallest buildings, which for weeks after the attacks spread a ghoulish dust over Lower Manhattan.

Visible progress in the roughly $11 billion project to rebuild the World Trade Centre site is now finally being made after suffering delays from political, security and financing concerns. The 1,776-foot (541-metre) centrepiece, 1 World Trade Centre, already stands more than 60 stories high.

September 11th Families for Peaceful Tomorrows urged Obama to now close the U.S. military prison housing foreign terrorism suspects at Guantanamo Bay in Cuba and bring home American troops in Afghanistan and Iraq.

"May the wreath you lay today, at the grave site of our loved ones, be more than a symbolic gesture," the group said in a statement.

The killing of bin Laden coincided with the first anniversary of a failed attempt to bomb New York's Times Square, one of at least 11 plots against the city that have been disrupted in the past decade.

Several recent polls showed Obama's job approval rating had been boosted after bin Laden's death although such bounces are often short lived, especially in the face of a difficult economy for many Americans. Obama's popularity ahead of the 2012 election where he is seeking a second term had been hurt by economic woes and high gasoline prices.