Pakistan's president acknowledged for the first time on Tuesday that his security forces were left out of a U.S. operation to kill Osama bin Laden, but he did little to dispel questions over how the al Qaeda leader was able to live in comfort near Islamabad.

The revelation that bin Laden had holed up in a compound in the military garrison town of Abbottabad, possibly for years, prompted many U.S. lawmakers to demand a review of the billions of dollars in aid Washington gives to nuclear-armed Pakistan.

"He was not anywhere we had anticipated he would be, but now he is gone," Pakistan President Asif Ali Zardari wrote in an opinion piece in the Washington Post, without offering further defense against accusations his security services should have known where bin Laden was hiding.

"Although the events of Sunday were not a joint operation, a decade of cooperation and partnership between the United States and Pakistan led up to the elimination of Osama bin Laden as a continuing threat to the civilized world."

It was the first substantive public comment by any Pakistani civilian or military leader on the airborne raid by U.S. special forces on bin Laden's compound in the early hours of Monday.

Pakistan has faced enormous international scrutiny since bin Laden was killed, with questions over whether its military and intelligence agencies were too incompetent to catch him or knew all along where he was hiding.

White House counterterrorism chief John Brennan told a briefing that Pakistan was not informed of the raid until after all U.S. aircraft were out of Pakistani airspace.

Senior U.S., Pakistani and Afghan officials later held a previously scheduled meeting in Islamabad to discuss the fight against militancy in Afghanistan and Pakistan but deflected questions about the bin Laden operation.

"Who did what is beside the point ... This issue of Osama bin Laden is history," Pakistani Foreign Secretary Salman Bashir told a joint news conference.

Marc Grossman, the U.S. special representative for Afghanistan and Pakistan, said both sides wanted to move beyond recriminations and finger-pointing.

But irate U.S. lawmakers earlier asked how it was possible for bin Laden to live in a populated area near a military training academy without anyone in authority knowing about it.

They said it was time to review aid to Pakistan. The U.S. Congress has approved $20 billion for Pakistan in direct aid and military reimbursements partly to help Islamabad fight militancy since bin Laden masterminded the September 11, 2001 attacks.

"Our government is in fiscal distress. To make contributions to a country that isn't going to be fully supportive is a problem for many," said Senate Intelligence Committee Chair Dianne Feinstein.

The White House acknowledged there was good reason for U.S. lawmakers, already doubtful of Pakistan's cooperation against al Qaeda, to demand to know whether bin Laden had been "hiding in plain sight" and to raise questions about U.S. aid to Islamabad.

"Certainly his location there outside of the capital raises questions. We are talking to the Pakistanis about this," said Brennan, adding it was 'inconceivable that bin Laden did not have a support system in the country that allowed him to remain there for an extended period of time'.

There were no protests and there was no extra security in Islamabad on Tuesday, just a sense of embarrassment or indifference that bin Laden had managed to lie low for so long in Abbottabad.

"The failure of Pakistan to detect the presence of the world's most-wanted man here is shocking," the daily News said in an editorial, reflecting the general tone in the media.

Zardari has made no address to the people of a country where anti-American sentiment runs high, prompting one Twitter user to tweet: "Most wanted man is killed on Pakistani soil and the Pres doesn't address his people, instead writes an op-ed for USA."

Pakistan has a long history of nurturing Islamist militants in the interests of its strategic objectives, primarily facing up to what it sees as its biggest threat -- India. Pakistan's fear of India has been at the root of its support for the Afghan Taliban and separatist militants in Indian Kashmir.

Warnings of revenge

In the first sign militants were attempting to strike back, Afghan forces killed and wounded 25 foreign fighters after they crossed the border from Pakistan, a government official said.

Jamaluddin Badr, governor of Afghanistan's northeastern Nuristan province, said the fighters killed overnight included Arabs, Chechens and Pakistanis.

Taliban, al Qaeda and other Islamist militants have long operated out of safe havens and training camps in Pakistan's largely lawless northwest Pashtun tribal regions. Bin Laden was sheltered by the Afghan Taliban before the September 11 attacks.

The United States earlier issued security warnings to Americans worldwide. CIA Director Leon Panetta said al Qaeda would 'almost certainly' try to avenge bin Laden's death.

Bin Laden's death had initially boosted the dollar and shares in the belief his killing reduced global security risks.

But shares dipped on Tuesday and the dollar struggled to pull away from a three-year low as markets refocused on a fragile global economy and corporate earnings prospects. Still, the threat of retaliatory attacks by al Qaeda could support oil prices, analysts said.

The body of the world's most powerful symbol of Islamist militancy was buried at sea after he was shot in the head and chest by U.S. special forces who were dropped inside his sprawling compound by Blackhawk helicopters.

Bin Laden, 54, was given a sea burial after Muslim funeral rites on a U.S. aircraft carrier, the Carl Vinson. His shrouded body was placed in a weighted bag and eased into the north Arabian Sea, the U.S. military said.

Analysts warned that objections from some Muslim clerics to the sea burial could stoke anti-American sentiment. The clerics questioned whether the United States followed proper Islamic tradition, saying Muslims should not be buried at sea unless they died during a voyage.

The U.S. administration was weighing whether to release a photo of bin Laden's body as proof that he had been killed. There was also a video of the sea burial but it was not clear if it would be released, a U.S. official said.

Night raid near Islamabad

Americans clamored for details about the secret U.S. military mission. A small U.S. strike team, dropped into bin Laden's heavily fortified hideout under the cover of night, shot the al Qaeda leader to death with a bullet to the head. He did not return fire.

Bin Laden's wife, originally thought killed, was wounded. Another woman was killed in the raid, along with one of bin Laden's sons, in the tense 40 minutes of fighting.

President Barack Obama and his staff followed the raid minute-by-minute via a live video feed in the White House situation room, and there was relief when the commandos, including members of the elite Navy SEALs, stormed the compound.

"We got him," the president said, according to Brennan, after the mission was over.

National Journal said U.S. authorities used intelligence about the compound to build a replica of it and use it for trial runs in early April.

Under bin Laden, al Qaeda militants struck targets from Indonesia to the European capitals of Madrid and London.

But it was the September 11 attacks, in which al Qaeda militants used hijacked planes to strike at economic and military symbols of American might and killed nearly 3,000 people, that brought bin Laden to global infamy.