An online posting attributed to al-Qaeda on Friday confirms the death of Osama bin Laden and warns of retaliation against the United States and other nations for the slaying of the terrorist leader.

The statement, which was posted on jihadist Web forums, appears to mark the first acknowledgment by al-Qaeda that its founder and leader for more than two decades was killed in a U.S. commando raid in a garrison city in Pakistan.

If confirmed, the message would also represent the network’s first attempt to appeal to its followers for continued commitment to its cause after the demise of the group’s most inspirational figure.

Bin Laden’s blood will be “a curse that chases the Americans and their agents, and goes after them inside and outside their countries,” the statement said, according to a translation by the SITE Intelligence Group, which tracks militant postings online.

“Soon — with help from Allah — their happiness will turn into sorrow, and their blood will be mixed with their tears. We will fulfill the oath of Sheikh Osama, may Allah have mercy on him: America and those who live in America will never enjoy security until our people in Palestine enjoy it.”

The origin of the message posted Friday is unclear. U.S. officials said that analysts at the CIA and other American spy agencies were evaluating the message and seeking to authenticate it.

“We can’t confirm at this point that it is authentic,” a U.S. official said. But the official said that a message from the network has been expected. “They have [done so] in the past when senior leaders have been killed.”

Bin Laden was killed at a compound in the Pakistani garrison city of Abbottabad early Monday morning in an operation by U.S. commandos that marked the culmination of a decade-long manhunt.

An acknowledgment from al-Qaeda could quell skepticism fueled by a lack of publicly accessible evidence that bin Laden was killed. His corpse was buried in the Arabian sea. And although the United States says it has conclusive DNA and photographic evidence that he was shot dead in the raid, President Obama has decided — at least for the time being — against the release of photographs of bin Laden’s body.

“We don’t trot out this stuff as trophies,” Obama said in an interview with “60 Minutes” scheduled to air Sunday night. “The fact of the matter is, you will not see bin Laden walking on this Earth again.”