A statement purporting to come from a son of Osama bin Laden denounced the al Qaeda leader's killing as "criminal" and said his burial at sea had humiliated the family, an online monitoring service said.
The statement, attributed to Omar bin Laden, bin Laden's fourth eldest son, said the al Qaeda chief's children reserved the right to take legal action in the United States and internationally to "determine the true fate of our vanished father", the SITE Intelligence Group said.
There was no independent confirmation of the authenticity of the letter, published on the website of Islamist ideologue Abu Walid al-Masri, although several specialists on militant propaganda said the text appeared genuine.
Omar bin Laden, who has been based in the Gulf in recent years, did not immediately respond to emailed and telephoned requests for comment.
The letter said, in part: "We hold the American President (Barack) Obama legally responsible to clarify the fate of our father, Osama bin Laden, for it is unacceptable, humanely and religiously, to dispose of a person with such importance and status among his people, by throwing his body into the sea in that way, which demeans and humiliates his family and his supporters and which challenges religious provisions and feelings of hundreds of millions of Muslims."
The letter said the U.S. administration had offered no proof to back up its account of the mission. It alleged the goal of raid had been to kill and not arrest, adding that afterwards the American commandos had "rushed to dispose of the body".
Some Muslims have misgivings about how U.S. forces killed bin Laden in a raid inPakistan on May 2 and disposed of his body in the ocean.
Bin Laden's swift burial at sea, in what many Muslims say was a violation of Islamic custom, has also stirred anger.