The official also said the domestic spy agency MI5 would examine possible connections between Friday's bomb attempt and at least two similar foiled plots - to attack a London nightclub in 2004 and to pack limousines with gas canisters and shrapnel.
In the 2004 plot, accused members of an al-Qaida-linked terror cell were convicted of conspiring to cause explosions. One of the possible targets M15 overheard them discussing was the Ministry of Sound, one of London's biggest and most famous nightclubs.
One man is heard saying the plan was to "Blow the whole thing up."
The discovery of the bomb resurrected fears that followed the July 7, 2005, suicide bombings that killed 52 people on three London subways and a bus and failed attacks on the transit system just two weeks later.
Gordon Brown, who only Wednesday succeeded Tony Blair as prime minister, called it a stark reminder that Britain faces a serious and continuous threat of terrorist attacks: "I will stress to the Cabinet that the vigilance must be maintained over the next few days."
Gordon Johndroe, the National Security Council spokesman, said American officials were in contact with their British counterparts "and will continue to monitor the situation."
The terror threat level in Britain has remained at "severe" - meaning a terrorist attack is highly likely - since last August.
On Friday, Metropolitan Police said it sent more officers on the streets of central London. Authorities also stepped up security at Wimbledon.
One analyst said the bombers could be trying to send Britain's new leader a message.
"It's a way of testing Gordon Brown," said Bob Ayers, a security expert at the Chatham House think tank. "It's not too far-fetched to assume it was designed to expedite the decision on withdrawal (from Iraq)."
Associated Press writer Raphael G. Satter contributed to this report.