The statement was posted by Web sites that have carried al Qaeda statements in the past. 

A U.S. official told CNN that Washington can't vouch for the authenticity of the claim, "but we do view it as credible. We're taking it very seriously." 

Last week's suicide bombing at an Israeli-owned resort hotel in Mombasa killed 10 Kenyans and three Israelis. A missile attack on an Israeli charter jet leaving Mombasa the same day was unsuccessful. No one aboard was hurt and the plane landed safely in Tel Aviv. 

"Al Qaeda announces officially it's behind the two attacks in Mombasa," the message said. "This statement comes as a challenge to the American enemy and to let it know it's capable of reaching anyplace in the world." 

Referring to the nearly simultaneous August 1998 bombings of U.S. embassies in Kenya and Tanzania, the message said last week's attacks came in the "same place that the crusading Jewish alliance was hit four years ago" by al Qaeda terrorists. 

The U.S. official said Kenya is a region the terrorist group knows well: "They've operated in it in the past. It shows their trademark signature in that the attacks were launched simultaneously." 

Paul Eedle, a London-based computer expert who monitors Web sites linked to the terrorist organization, said the statement appeared on sites that regularly carry al Qaeda pronouncements. 

"It says very directly to Israelis that as you kill our children, we will kill your children," Eedle said. 

U.S. officials said Monday the shoulder-fired missiles used to target the charter jet apparently came from the same batch as one used in a failed attempt to shoot down a U.S. military plane near the Prince Sultan Air Base in Saudi Arabia last May. 

Serial numbers on the two weapons were close, according to sources. That "strongly suggests" a link to al Qaeda, said a U.S. official, who stopped short of calling the evidence conclusive. 

U.S. intelligence officials said they believe they know where and when al Qaeda operatives obtained the Soviet era SA-7s, which have an effective range of 10,000 to 12,000 feet. 

Israeli sources said they believe the attacks were launched from Somalia, Kenya's largely lawless northern neighbor, and were organized by Fazul Abdullah Mohammed -- whom Israeli officials identified as al Qaeda's chief of operations there. 

Mohammed is under U.S. indictment in the embassy bombings in Nairobi, Kenya, and Dar es Salaam, Tanzania. 

In Mombasa, Kenyan officials were considering an Israeli request to take key pieces of evidence from the hotel bombing to Israel for analysis. 

The Israelis want parts of two gas-welding cylinders believed to have been packed with 200 kilograms of explosives, charred remains of an AK-47 assault rifle found in the rubble of the hotel and launchers used to fire the missiles on the Arkia Boeing 757 charter flight. 

Israeli agents were deeply involved in the probe, including those from the country's intelligence service, Mossad, dispatched by Prime Minister Ariel Sharon. 

Israeli and Kenyan investigators were said to have engaged in heated discussions about who would carry out the forensic investigation. 

Israeli investigators believe they stand a better chance of identifying the bombers because they have better forensic resources and more experience in handling bombings. 

The Kenyans sought to hold the evidence, at least initially, but were willing to pass it along to Israel if more sophisticated analysis was needed, Kenyan authorities said. A decision could ultimately be made by Kenyan President Daniel arap Moi. 

Kenyan authorities quickly detained 12 people for questioning. They released two people, including an American, but were still holding six Pakistanis and four Somalis. 

Authorities said the 10 had entered the country by boat and did not have proper travel documents. Most had passports issued in Somalia, Kenyan officials said, but apparently they were not involved in the attacks. 

A senior Bush administration source told CNN that al Qaeda and a Somali-based Islamist group, Al-Ittihad al-Islami, or AIAI, topped the list of suspects. The official and other U.S. officials said they believe AIAI is associated with al Qaeda. 

As a result of the Kenyan attacks, Israel believes it is now part of the global war on terrorism. 

"We don't choose the battleground. We don't choose the enemy. They choose us. And we are now involved," a senior Israeli official. 

Ephraim Halevy, former head of Mossad and currently Sharon's special liaison for Iraq issues, addressed the Kenyan attacks at a national conference on security Monday in Herzliya, Israel.. 

He said it would be "correct" to understand what happened in Mombasa as if the scenario resulted in an even larger tragedy -- if the planes would have been shot down too -- to "judge the new situation on this basis." 

"The realization of a mega-terrorist attack against Israel by international jihad or by other terrorist organizations changes the rules of the game," Halevy said. 

"It changes the national spirit and creates international dynamics which open before us new options that until now were unacceptable by international public opinion."