Captain Akbar Naqi, a navy spokesman, said: "It looks like a suicide attack on the naval war college."
Abduyllah Yasim, a local police official, said: " There were three blasts and many casualties are feared."
Witnesses said two of the three blasts were triggered by the bombers. The third explosion occurred when cannisters left by the bombers exploded.
Frequent attacks
Pakistan has been hit by a series of attacks on security forces and military installations in recent months.

A teenage bomber blew himself up at a meeting of tribal elders discussing how to tackle al-Qaeda and Taliban fighters in the northwestern town of Darra Adam Khel on Sunday, killing 43 people.

On Friday, a suicide bomber killed at least 44 people in the northwestern Swat valley at the funeral for a policeman killed in a roadside bombing. On Saturday, a suicide car bomber killed two people in the lawless Bajaur tribal district.

The army's top medical officer, Lieutenant General Mushtaq Baig, was killed in a suicide attack in the garrison city of Rawalpindi on February 25.