Washington: Warning terror groups not to mess up with it, the US has said the decade after the 9/11 attacks has shown to the world the Americans' national resolve to bring the enemy responsible to justice.

"The people who attacked us on 9/11 were trying to weaken America, were trying to hurt America -- and instead they strengthened us, because you don't mess with this country when you attack us," Defence Secretary Leon Panetta.

"What we made clear is that when that happens, we will come and get you," Panetta said on the eve of the 10th anniversary of the 9/11 attacks. "This country unified as a result of what happened."

He said when he was stuck in Washington days after 9/11, he rented a car to go to his home in California. "I decided I had to get -- rent a car and get back home, so I rented a car, drove across country to get back to California."

"And not only did I do that in record time -- but I have to tell you, what I witnessed that day driving across the country is something that's also seared into my memory, because what I witnessed was this country coming together.

"As I was driving across the country, there were signs coming up, 'God bless America,' there were flags that people were putting up; people going to churches, people holding hands. You could see this country coming together to try to confront what had happened," Panetta said.

It brought this country together in a way that is most reflected by people who put their lives on the line of duty every day and it is reflected by the people in the military, men and women who put their lives on the line of duty every day in the service of this country, he said.

"The greatest thing this country has are people who are willing to put their lives on the line in order to protect others," Panetta said.

"We not only commemorate those nearly 3,000 innocent lives who perished on September 11, 2001, but we also honour those who stepped forward in the wake of those attacks, the generation that answered the nation's call to serve in a time of war ... for a better life, a better America and a better world," he said.

Panetta said he is the son of Italian immigrants who, like millions of others, came to this country with few skills, little money in their pockets and hardly any-English speaking ability.

"But they understood the dream of America. When I would ask my father, why you traveled those thousands of miles to come to a strange country, I will never forget his reply: 'because your mother and I believed that we could give our children a better life'," he said.

Since 9/11, a new generation of Americans joined the ranks by the millions, knowing fully well they would likely be sent into battle, he said.

"For ten long years they have fought, and died, in places such as Fallujah and Sadr City in Iraq, and in remote outposts in Afghanistan's Helmand and Korengal valleys. In Iraq's city streets and in Afghanistan's mountains, this generation has spilled its blood so that their fellow citizens and future generations will have a safer and a better life," he said.

"When I visited our troops in Afghanistan and Iraq, I looked into the eyes of those men and women who serve our country on the frontlines. So many of them were motivated to do so by the events of September 11, 2001. In their eyes I saw fierce determination to carry out their mission of protecting America," Panetta said.

"It was that determination that brought us our greatest victory against the perpetrators of the 9/11 attacks, with the operation that brought Osama bin Laden to justice" on May 2, he said.