US Defense Secretary Leon Panetta hailed the "fierce determination" that emerged from the attacks of September 11, 2001, praising those who volunteered to serve in uniform as the country marked the 11th anniversary of the tragedy.

Speaking at a ceremony honoring the 184 people who died when a hijacked airliner slammed into the Pentagon, Panetta said the attacks failed to weaken America and instead inspired a generation to take up arms.

"For today we also recall that out of the shock and sadness of 9/11 came a new sense of unity and resolve, that this would not happen again," Panetta told families of those who died in the Pentagon attack.

"It inspired a fierce determination to fight back and protect our way of life."

Panetta was joined at the ceremony by President Barack Obama, who placed a floral wreath at the memorial marking the spot where Flight 77 hit that day 11 years ago at 9:37 a.m.

An ensemble of US Navy musicians dressed in crisp, white uniforms played patriotic hymns in somber tones as Obama spoke to relatives of those who died at the Pentagon.

The attacks were a watershed for the country and for the US military, which transformed its tactics but struggled to sustain an all-volunteer force that has been stretched to the breaking point after a decade of war, amid an epidemic of suicides and traumatic brain injuries.

"In trying to attack our strengths, the terrorists unleashed our greatest strength: the spirit and the will of Americans to fight for their country," said Panetta.

"Millions of Americans responded. A whole new and great generation stepped forward to serve in uniform, to fight this war on terrorism. They bled on distant battlefields. They relentlessly pursued those who would do us harm," he said.

The Pentagon chief, who as CIA director presided over the successful raid last year that killed Osama bin Laden, said the United States relentlessly pursued the Al-Qaeda leader.

"We never gave up the search for bin Laden. We successfully brought him to justice," he said.

Since the September 11 attacks, US military forces and intelligence agencies have waged war -- openly and covertly -- on Al-Qaeda and its affiliates in Pakistan, Afghanistan, Yemen, Somalia and elsewhere.

An invasion of Iraq launched by ex-president George W. Bush in 2003 had no direct connection to Al-Qaeda, though administration officials argued at the time Saddam Hussein's regime was a dangerous adversary that had to be disarmed.

In the chaotic aftermath that followed Saddam's fall from power, a lethal wing of Al-Qaeda rose up in Iraq.

US troops have since withdrawn from Iraq, while Obama has ordered a gradual drawdown in Afghanistan, with the bulk of combat forces due to depart by the end of 2014.

The grinding, 11-year-old war in Afghanistan appears at a stalemate amid flagging popular support for the conflict on both sides of the Atlantic. But Panetta said the war effort was making progress.

American troops have denied Al-Qaeda and its allies a safe haven in Afghanistan, said Panetta, adding that "they're fighting so that Afghanistan can secure and govern itself."