ISLAMABAD, Pakistan — Pakistani military officials struggled unsuccessfully on Monday to explain how a small team of insurgents managed to scale the back wall of one of the country’s premier naval air stations and destroy two American-provided surveillance planes. Before the fighting was over, the attackers had killed at least 10 Pakistani security officers in a gun battle that lasted for hours and raised new questions about the competence of the country’s armed forces.

The attack on the Mehran naval air base, just off of one of the main boulevards in the southern port city of Karachi, stunned the country as it was still questioning how an American commando team entered Pakistani airspace unnoticed on the mission to kill Osama Bin Laden this month.

Pakistan’s military, which was just initiating efforts to rebuild its credibility after that episode, and preparing for possible retaliatory raids by the Taliban or affiliates of Al Qaeda, insisted that its bases — especially those containing nuclear weapons — were highly secure. There is no evidence that any nuclear weapons were kept in the base’s arsenal, though they are believed to be stored in large numbers about 15 miles away.

The criticism was focused on the chief of the Pakistani Navy. Journalists and retired service members repeatedly questioned the Naval chief, Adm. Nauman Bashir, for denying that there was a major security lapse.