A loud blast alarmed residents and security officials in the Pakistani capital, Islamabad, and nearby city of Rawalpindi on Monday but police later said it appeared it had been a sonic boom.

Residents of both cities heard a big blast just before 11 a.m. (6 a.m. British time).

"We have checked everywhere in Rawalpindi, all the main areas and hospitals, but there is nothing," a senior city police official said about 45 minutes after the blast was heard.

"It could have been a sonic boom but we are still investigating."

A Pakistani air force spokesmen said none of their aircraft had been over the city at the time of the blast.

A military official said there had been no bomb at any of the main military installations in Rawalpindi, the headquarters of the Pakistani army and the home of President Pervez Musharraf.

Militants have launched several bomb attacks in both Rawalpindi and Islamabad over the past year.

The Taliban militant commander held responsible for most of the attacks threatened on the weekend to retaliate for a government offensive against militants in a northwestern region on the Afghan border.