The United States confirmed that an American vehicle was attacked in the city of Peshawar in northwest Pakistan on Monday, wounding two American and two Pakistani consulate staff.
The information minister of Khyber Pakhtunkhwa province, Mian Iftikhar Hussain, insisted in the regional parliament that two Americans were also killed, without giving any further details on their identity.
A US state department statement referred only to staff working for the US consulate in Peshawar.
Spokeswoman Victoria Nuland said no US consulate staff were killed, but said the United States was "seeking further information about other victims of this heinous act".
"We can confirm that a vehicle belonging to the US consulate in Peshawar was hit in an apparent terrorist attack," she said in a statement released by the embassy in Islamabad.
She said two US personnel and two Pakistani staff of the consulate were receiving medical treatment after being wounded in the attack.
"We stand ready to work with Pakistani authorities on a full investigation so that the perpetrators can be brought to justice," she added.
A suicide bomber rammed his vehicle into another near a building occupied by the UN refugee agency Peshawar on Monday, killing five people, including two Americans, Hussain had said.
There was no immediate claim of responsibility for the attack, which left a crater along a busy street. Firemen extinguished flames of a vehicle that was mangled and blackened from the blast.
He said five people were killed and that the Americans had been moving across the city with security guards.
"The blast killed two Americans. This is a dangerous move from the terrorists -- they want to terrorise the foreigners," Hussain told AFP, adding that two Americans were also wounded in the blast.
"The dead and injured are from the US consulate. It was a suicide attack. The suicide bomber targeted US vehicle," he said.
Police have recovered a half-burnt US passport from a car badly damaged in the explosion, intelligence officials said.
An intelligence official confirmed the US consulate car was the target, with another saying the suicide bomber ploughed his vehicle into it.
Peshawar police chief Imtiaz Altaf said the car bomb was loaded with up to 110 kilograms (240 pounds) of explosives, including more than 10 mortar rounds. There were 19 wounded in the attack, he said.
Television stations repeatedly broadcast an image of a US passport at the scene, its corners burned by the flames.
Pakistan's Taliban, who are close to al Qaeda, are blamed for many of the suicide bombings across Pakistan, a strategic US ally.
Those attacks had eased in recent months but it was not clear if the lull was due to pressure from military offensives or a shift in tactics.
(With AFP and Reuters inputs)