Stephen Hadley, Bush's National Security Adviser, told Fox News on Sunday: "We have seen those reports out of Pakistan. Obviously, we're looking into them.
"At this point, we are not in a position publicly to confirm that he is dead."
He said Rabia, an Egyptian, had taken over as head of operations for al-Qaida after the capture in May of Abu Faraj al-Libbi, described as the number three in al-Qaida after Osama bin Laden and Ayman al-Zawahiri.
"Hamza Rabia is a bad guy," Hadley said.
But his body has not been found, and confirmation of his death is based on intelligence reports and message intercepts, intelligence sources say.
Officials say Rabia's corpse, along with those of two comrades, was removed by fighters and buried.
Adding to the speculation on Sunday, Pakistani tribesmen displayed parts of a missile they said hit a house and killed two boys. One piece of casing bore the words "US" and, in capitals, "missile".
Sitting amid the ruins of his home in North Waziristan, Haji Mohammad Siddiq told Reuters that his 17-year-old son and an eight-year-old nephew were killed in a missile attack.
"When I went there I saw my son, Abdul Wasit, and my eight-year-old nephew, Noor Aziz, were dead"
Haji Mohammad Siddiq
"There were no foreigners in my house," Siddiq said. "I have nothing to do with foreigners or al-Qaida.
"We were sleeping when I heard two explosions in my guest room. When I went there I saw my son, Abdul Wasit, and my eight-year-old nephew, Noor Aziz, were dead.
"I heard more explosions and went out to the courtyard, and when I looked up at the sky, I saw a white drone. I saw a flash of light come from the drone followed by explosions."
Al-Arabiya, the Dubai-based satellite TV channel, said on Saturday it had been contacted by a person claiming to be from al-Qaida who denied that Rabia had been killed.
An Al-Arabiya presenter told viewers that the caller had said five people had been killed in an explosion in the tribal region, but these were two local men, two Tadjiks and an Arab called Sulayman al-Maghrabi.
According to Hadley, Rabia was involved in planning two assassination plots against Pervez Musharraf, the Pakistani president.
"So, if he has been killed, that's a good thing for the war on terror," he said. "It's part of the effort to kill or capture the major al-Qaida leadership."
Abu Faraj Farj al-Libbi was
captured in May 2005
Musharraf said on Saturday that Rabia had been killed in North Waziristan, a tribal area of Pakistan on the Afghan border.
"Yes indeed, 200% confirmed," Musharraf told reporters while in Kuwait as part of a tour of the Middle East.
According to US media reports, Rabia was killed by a missile fired by an unmanned Predator drone controlled by the US Central Intelligence Agency.
Pakistani officials said that Rabia had been killed in an explosion.
Several attempts have been
made to assassinate Musharraf
Asked by Fox whether the US had helped "take out" Rabia, Hadley said: "We've obviously been supporting Pakistan.
"President Musharraf has been very aggressive in dealing with the al-Qaida and Taliban presence in Pakistan.
"We have helped him in terms of providing intelligence and co-operating with his forces, and obviously this is something that would be an important thing for Pakistan, an important thing for the United States."
Pakistan, sensitive to domestic public opinion, has denied US drone aircraft have carried out missile strikes on its soil in the past and Washington has declined to comment.