The Pakistani air force said a fighter jet crossed the sound barrier outside the capital on Monday, causing a sonic boom whose two loud blasts were heard for miles and spurred fears of a militant attack.

The morning blasts echoed in Rawalpindi and the capital, Islamabad, two cities about 7 miles apart. Rawalpindi houses the headquarters of Pakistan's army and the residence of President Pervez Musharraf, a key U.S. ally in the war on terror.

A statement hours later from the air force said the F-7P aircraft was flying in the Pindigheb area — about 55 miles southwest of the capital — and shifting from an altitude of 40,000 to 15,000 feet.

The resulting sonic boom waves traveled a lengthy distance due to the clear weather conditions as well as the high altitudes involved, the statement said.

No damage was reported, but the noise set the cities on edge as it came amid a military operation against extremists near the Afghan border that has spurred Taliban promises of revenge.