A car filled with explosives rammed into a US government vehicle in the north-western Pakistani city of Peshawar, killing two people and wounding 19 others, according to police.
The identities of those who died in the attack were unclear, said local police officer Pervez Khan, but he added that some of the wounded were foreigners.
A US passport was found in the wreckage, said another police officer, Javed Khan. The US embassy in Islamabad said it was investigating the report.
Peshawar is located near Pakistan's semi-autonomous tribal region, the main sanctuary for Taliban and al-Qaida militants in the country. The city has been hit by scores of bombings in recent years, but attacks against US targets are relatively rare.
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 Pervez Khan, who was part of the police escort providing security for the vehicle.
Local TV footage showed an SUV at the site that had been completely destroyed. All that was left was a carcass of blackened, twisted metal. A police explosives expert, Abdul Haq, said 110 kg (240lb) of explosives were used in the incident.
Irfan Khan, a local resident, said he was at a nearby shop when the blast occurred.
"I quickly looked back in panic to see smoke and dust erupt from the scene," he said. "I ran toward the scene along with others and saw two vehicles destroyed and the larger vehicle on fire."
One dead person was on the ground near the SUV, and a foreigner was injured, said Irfan Khan.
"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.
Another vehicle arrived and took away the injured foreigners, said Javed Khan.
Some of the policemen escorting the US vehicle were also wounded in the attack, and their vehicle was damaged, he added.