Four people -- including an American diplomat -- were killed and 17 others injured when a car bomb exploded near the highly fortified US consulate in Karachi on Thursday morning.
The explosion, which comes two days before United States President George W Bush's visit to Pakistan, occurred at about 9 am local time in the parking area of the Marriott Hotel, almost next door to the consulate building.
President Bush is now in India on a three-day visit.
Pakistani Information Minister Sheikh Rashid Ahmad said it was too early to say who could be behind the blast.
Provincial police chief Jahangir Mirza did not rule out the possibility of a suicide attack, but said, "We are still investigating."
Local channels said the explosion occurred when a van of the paramilitary vehicle of the Rangers tried to stop a suspected suicide bomber from getting close to the US consulate through one of the entrances of Marriott Hotel. This resulted in the death of a Ranger's soldier and the destruction of a security vehicle.
Security in Islamabad, already beefed up for Bush's visit, has further been boosted with paramilitary forces being deployed at all sensitive locations.
Bush was expected to land at the Chaklala Air Base in Rawalpindi when he arrives in Pakistan from India on Saturday.
Officials and analysts said the blasts could be in retaliation to Bush's comments in Kabul on Wednesday that Al Qaeda chief Osama bin Laden and Taliban chief Mullah Omar will be brought to justice.
Officials said no militant outfit has claimed responsibility so far.
Complete Coverage: The Bush Visit