Al-Qaeda confirmed Friday the killing of Osama bin Laden and warned of retaliation, saying Americans' "happiness will turn to sadness."

The confirmation came in an internet statement posted on militant websites, signed by "the general leadership" of al-Qaeda. The announcement opens the way for the group to name a successor to bin Laden. His deputy Ayman al-Zawahri is now the most prominent figure in the group and is a very likely contender to take his place.

White House spokesman Jay Carney said the Obama administration is aware of the statement.

"What it does obviously is acknowledge the obvious, which is that Osama bin Laden was killed on Sunday night by U.S. forces.… We're quite aware of the potential for [terrorist] activity and are highly vigilant on that matter for that reason."

The statement, dated May 3, was the first by the group since bin Laden was killed on Monday, Pakistan time, by U.S. commandos in a raid on his hideout in Abbottabad, Pakistan. The statement's authenticity could not be independently confirmed, but it was posted on websites where the group traditionally puts out its messages.

"We stress that the blood of the holy warrior sheik, Osama bin Laden, God bless him, is precious to us and to all Muslims and will no go in vain," the statement said. "We will remain, God willing, a curse chasing the Americans and their agents, following them outside and inside their countries."

"Soon, God willing, their happiness will turn to sadness," it said, "their blood will be mingled with their tears."

Islamists holding a picture of Osama bin Laden chant pro-bin Laden and anti-American slogans in Cairo Friday. (Asmaa Waguih/Reuters)

About 300 Islamists marched in Cairo Friday, protesting the U.S. killing of bin Laden. The crowd was blocked by Egyptian soldiers when it tried to get to the U.S. Embassy. Marchers shouted at diplomats inside, saying, "leave, leave" and "lower the [U.S.] flag."

There was no indication how al-Qaeda might retaliate. Rather than making vehement cries of vengeance, the statement — entitled "You lived as a good man, you died as a martyr" — struck a tone of calmness and continuation. Though it included praise of bin Laden, much of the 11-paragraph statement was dedicated to underlining that al-Qaeda would live on, depicting him as just another in a line of "martyrs" from the group.

"It is impossible, impossible. Sheik Osama didn't build an organization to die when he dies," the statement read. "The university of faith, Quran and jihad from which bin Laden graduated will not close its doors," it added.

"The soldiers of Islam will continue in groups and united, plotting and planning without getting bored, tired, with determination, without giving up until striking a blow," the statement said.

It said bin Laden was killed "along an established path followed by the best of those who came before him and those who will come after him."

Apparently unaware of burial at sea

In the statement, al-Qaeda also called on Pakistanis to rise up in revolt against its leaders to "cleanse the shame." It also said that an audio message bin Laden recorded a week before his death would be issued soon.

The writers of the statement appeared unaware of the announcement by U.S. officials that bin Laden's body had been buried at sea. The statement warned against mishandling or mistreating bin Laden's body and demanded that it be handed over to his family, saying "any harm [to the body] will open more doors of evil, and there will be no one to blame but yourselves."

The statement came five days after President Barack Obama announced bin Laden's death Sunday night, Washington time, in a U.S. raid in Pakistan.