Speaking ahead of the 10th anniversary of the September 11 attacks that triggered the Afghan war, Ban also recalled the chaos of that fateful morning which he witnessed first hand from the UN headquarters in Manhattan.

The UN secretary-general stressed that while foreign combat troops are due to leave Afghanistan by 2014, this should not be seen as a full exit for the global community.

"The international community has an obligation to continuously engage with Afghanistan even though ISAF (International Security Assistance Force) members may withdraw their military engagement," he said in Canberra.

"There is very serious insecurity," Ban added, saying that foreign forces could help strengthen the Afghan National Army and police.

He said the UN could provide technical assistance and political advice to the administration of President Hamid Karzai to conduct negotiations with Taliban-led insurgents, which Western officials say are tentatively underway.

"The military means are important and useful and can be effective but that's not all," he said.

"There should always be a political dialogue and there is a consensus in the international community that there needs to be some sort of negotiation and dialogue."

There are currently around 100,000 US troops in Afghanistan, of which 33,000 will leave by mid-2012 in a process scheduled to see all American combat soldiers exit the country by the end of 2014.

Ban's comments came as US officials Thursday warned of a fresh potential terror strike on the United States, reportedly involving bomb-laden vehicles as Americans braced to mourn those killed in the 9/11 tragedy.

Authorities have warned of a possible backlash following Al-Qaeda founder Osama bin Laden's killing by US forces in May.

Ban, who was in New York the day two passenger jets slammed into the World Trade Center a decade ago, bringing down the twin towers, said he remembered the horror and confusion of those first few hours.

"I myself was at the scene on that particular day," he said.

"I was going to be just in a few hours time the chief of staff of the president of the general assembly.

"Because of the terrorist attacks, everybody had to be evacuated. There was a vacuum of leadership in the United Nations. It was quite a chaotic situation for the United Nations.

"After 24 hours we convened the general assembly. This is still very vivid in my memory. The first thing we did... was adopt the strongest possible resolution of the general assembly... condemning terrorist attacks in the strongest possible terms."

But Ban said despite "resolutions, declarations and efforts, the international community has not been free from these attacks" in the decade since, noting last month's bombing of the UN mission in Abuja, Nigeria.

"That was the biggest loss in the history of the United Nations. Twenty-three UN staff killed and more than 80 people wounded," the secretary-general said.

"My position is that terrorism can not be justified under any circumstances or whatever justification. This must be stopped."

Ban, who will return to New York Friday to attend a concert to mark the anniversary, is the first UN chief to visit Australia since Kofi Annan toured in 2000.