WASHINGTON (AFP) - US Defense Secretary Robert Gates vowed Tuesday on the sixth anniversary of the September 11 attacks to relentlessly hunt down Al-Qaeda leaders amid new warnings of a heightened threat of attack.

Gates joined relatives and dignitaries in a moment of silence at the time and place where a hijacked American Airlines jet slammed into the Pentagon, killing 184 people as part of the deadliest terror attack on US soil.

Alluding to threat warnings by US intelligence chiefs, Gates said "anyone wishing to revisit harm upon this country will find, in the men and women in this department, adversaries who have found a clarity of purpose in their grief, a strength of resolve in their anger."

"The enemies of America -- the enemies of our values -- will never again rest easily for we will hunt them down relentlessly and without reservations," he said.

The Pentagon, one side of which was left a fiery ruin by the attack, has since been rebuilt but its victims have not been forgotten, Gates said.

"You have never been and never will be alone in your sorrow," he said.

An air force bugler played taps and relatives were invited to visit the site of a memorial being built for the victims.

Fifty-nine of the victims were aboard American Airlines Flight 77, which was hijacked by five men after takeoff from Dulles International Airport and flown into the southwest side of the building.

Another 125 people were killed inside the Pentagon as the aircraft exploded through the side of building.

Meanwhile, US intelligence has warned that Al-Qaeda has reconstituted its safe havens in Pakistan and is actively plotting new attacks against the United States.

Al-Qaeda leader Osama bin Laden reminded Americans that he is still alive earlier this week with a video taped message that vowed a US defeat in Iraq.

"We judge that the United States currently is in a heightened threat environment," Admiral Michael McConnell, the national intelligence director, said Monday.

The anniversary was overshadowed by congressional hearings across Washington over the unpopular Iraq.

General Peter Pace, the chairman of the Joint Chiefs of Staff, said the anniversary was an occasion for the US military to recommit itself to defend the United States "against those who on this day declared war on US."

But Pace also alluded to the political divisions that have arisen over the US invasion of Iraq in 2003, which US leaders justified as necessary to prevent weapons of mass destruction from falling into the hands of Islamic extremists.

"There is a dialogue in this democracy as there should be," Pace said. "The dialogue rightly focuses on how, when and where we will defend against them."