Two people have died after a car filled with explosives rammed into a US government vehicle in the north-western Pakistani city of Peshawar, police said.
The dead were from Pakistan, while two foreigners were among the 19 people wounded in the attack, senior police officer Javed Khan said.
Their nationalities were not immediately known, but a US passport was found in the wreckage of the vehicle that was attacked, he said.
The US Embassy in Islamabad said it is investigating the incident.
Peshawar is located near Pakistan's semi-autonomous tribal region, the main sanctuary for Taliban and al-Qa'ida militants in the country.
The city has been hit by scores of bombings in recent years, but attacks against American targets are relatively rare, likely because of the extensive security measures taken by the US government.
The vehicle was attacked after it left the US Consulate in Peshawar and was travelling through an area of the city that hosts various international organisations, including the United Nations, said police officer Pervez Khan, who was part of the security escort for the vehicle as it moved.
Local TV footage showed an SUV at the site that was completely destroyed. Mr Khan said the images were of the US vehicle that was attacked.
A police explosives expert, Abdul Haq, said 240lbs of explosives was used in the attack.
One dead person was on the ground near the SUV, and a foreigner was injured, said Mr Khan.
He added: "We put the injured man and the dead body in a private vehicle. There were more injured in the surrounding area, too."
Another eyewitness, Wajid Ali, said he helped put another seriously injured foreigner into the vehicle.
But another vehicle arrived, presumably from the US Consulate, and took away the injured foreigners, said Javed Khan, the police officer.
Some of the policemen escorting the US vehicle were also wounded in the attack, and their vehicle was damaged, said Mr Khan.
US Secretary of State Hillary Rodham Clinton, on an Asian tour, condemned the suicide bomber for ramming his car into a US government vehicle, injuring two Americans.
Mrs Clinton, in the midst of an Asian tour, said at the outset of a news conference in Jakarta that she wants to "very clearly condemn the attack on our consulate personnel in Peshawar,Pakistan."
Mrs Clinton added: "We pray for the safe recovery of both American and Pakistani victims and once again we deplore the cowardly act of suicide bombing and terrorism that has affected so many around the world."
She was in Indonesia's capital to offer US support for a regionally endorsed plan to ease rising tensions by implementing a code of conduct for all claimants to disputed islands.