Reports of the number of people killed in the attack varied, with t
he Reuters news agency reporting at least 15 people were killed.
Police officials told the Associated Press news agency that at least 35 people were killed in the attacks, while s
ecurity officials told the AFP news agency at least 16 had died
Witnesses said a burned-out bus could be seen parked just inside the gates of the military spy agency, with dozens of ambulances stationed around.
"There were 50 people sitting in the bus. Many are injured, many are OK," army spokesman Major-General Waheed Arshad said.
The blasts come amid political turmoil in Pakistan, which is still under emergency rule imposed by General Pervez Musharraf in what has widely been seen as an attempt to safeguard his presidency from challenges to his re-election.
Twin suicide blasts in the city in September killed 25 people and wounded 70, many of them aboard an intelligence services bus.
Another suicide attack killed 15 soldiers near the capital in the same month.
A attacker blew himself up near Musharraf's army residence in Rawalpindi, killing seven people, in October.