Pakistani officials said at least 12 security forces were killed. It's unclear how many militants were killed.
Between 10 and 15 insurgents armed with grenades, rockets and automatic weapons raided Naval Station Mehran late Sunday and destroyed two U.S.-supplied surveillance aircraft.
Six Americans on the base at the time of the attack escaped unharmed, the U.S. Embassy said.
The Pakistani Taliban is claiming responsibility, saying it's revenge for the May 2 raid that killed Osama bin Laden.
The unilateral U.S. raid on bin Laden's compound in the northwest Pakistani garrison city of Abbottabad has triggered a strong backlash against Washington, as well as rare domestic criticism of the armed forces for failing to detect or prevent the American operation.
Pakistani leaders insist they had no idea the al-Qaeda boss had been hiding in Abbottabad.
This is the third major attack the group has claimed since the bin Laden killing. The others were a car bombing that slightly injured American consulate workers in the northwest city of Peshawar and a twin-suicide attack that killed around 90 Pakistani paramilitary police recruits.
The Associated Press contributed to this report.