Islamist militants launched a brazen assault on a fortified Pakistani naval air base in the southern port city of Karachi late Sunday, destroying a U.S.-supplied airplane and triggering blazing explosions and a long gun battle with Pakistani commandos.

The siege was still going on five hours after it began, and conflicting reports were emerging from the sealed base about the sequence of events and casualties. Some reports said foreigners, possibly Americans, were killed or taken hostage in the attack; a Pakistani navy spokesman said that five Pakistani security personnel were killed and that no foreigners had been on the base. A U.S. Embassy spokesman also said that no Americans were present.

The band of about 15 heavily armed militants entered through either a sewage stream or an air force museum near the base, local media reported. The Pakistani Taliban claimed responsibility for the attack, which apparently was targeted at a hangar holding an anti-submarine plane supplied by the United States, the PC-3 Orion. The hangar and the plane were destroyed, according to television station Express 24/7.

The Karachi attack was the second major assault on Pakistani security forces since Osama bin Laden was killed May 2, and it was the most brazen since gunmen stormed and held hostages at the military’s central headquarters in 2009. The naval base raid will probably be viewed as a particular embarrassment for the Pakistani military, revealing its weaknesses following insurgents’ vows to avenge bin Laden’s death by striking Pakistani and American targets.

The Pakistani armed forces faced harsh domestic criticism over the U.S. operation that killed bin Laden, which exposed the army as vulnerable to incursion by a foreign military and unable to locate the world’s most-wanted terrorist.

Sunday’s coordinated attack began about 10:30 p.m. with a trio of blasts that set several buildings ablaze. At least five more explosions occurred as Pakistani commandos and marines engaged in several firefights with the militants over several hours.

The implications for the Pakistani military’s capabilities would be even graver if the attackers were found to be aided from within the base, as some security analysts suggested.

“This seems to be an attack in which the terrorists were very well-informed,” Razi ud-Din Sheik, a defense analyst, told Express 24/7, adding that there must have been “information that was provided from inside to be able to do what was done.”

This was the second attack on the Pakistani navy in Karachi in recent weeks, representing what could be a new pattern for militants, who far more often strike security forces in the volatile northwest. Last month, the Pakistani Taliban claimed responsibility for deadly bombings on buses carrying naval employees to work in Karachi.