A suicide bomber killed himself and two others when he drove a car bomb into a US consular vehicle in the Pakistani city of Peshawar, say police.
US officials said no-one from the US consulate was killed, but that two US and two Pakistani staff were injured.
Earlier the Pakistani provincial information minister said two Americans were dead.
At least 19 people were injured when the bomber rammed the explosives-laden car into the four-wheel-drive vehicle.
Peshawar is a key city linking the volatile tribal areas bordering Afghanistan with the rest of Pakistan.
Taliban and al-Qaeda militants operate in the area and the city has been targeted frequently in bombings in recent years - although US officials are said to be protected by extensive security measures.
No group has yet claimed responsibility for Monday's attack.
'Smoke and dust'
Witnesses said the bomber's car rammed the US vehicle in Abdara Road area - close to the offices of the UN refugee agency, the UNHCR, and a registration centre for Afghan refugees.
The car driven by the suicide bomber contained about 110kg (240lb) of explosives, Pakistani police officer Abdul Haq said according to Associated Press (AP) news agency, and left a substantial crater at the blast site on the busy street.
Police said the bomber killed one person inside the targeted vehicle, and one passerby, as well as himself.
Irfan Khan, a local resident, told AP 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."
Earlier, provincial information minister Mian Iftikhar Hussain told AFP news agency that two Americans had been killed in the attack, which was an attempt to "to terrorise the foreigners".
A local journalist who saw a US passport inside the targeted four-wheel-drive said it carried the stamp of the US consulate and was in the name of an apparent employee of the consulate.
But in a statement released by the US embassy in Islamabad, state department spokesman Victoria Nuland said no consulate staff had been killed, but the US 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 the statement.
She said the two American and two Pakistani injured staff members were receiving treatment.
"We stand ready to work with Pakistani authorities on a full investigation so that the perpetrators can be brought to justice," she added.
Peshawar is one of the most dangerous cities in Pakistan for foreigners, with a number killed and injured in attacks there over recent years - although the level of violence has dropped somewhat since 2009-10, when attacks were reported almost daily.
In April 2010 the Taliban launched a gun and bomb attack on the consulate itself, killing at least seven people. There were no reported US casualties.
In May the following year militants bombed a US consulate convoy in Peshawar, killing one Pakistani and wounding 10 others. In August 2008 one of the diplomats at the consulate survived an attack on her armoured vehicle.
Peshawar is the easiest urban target for militants from the tribal areas to strike, and diplomats and aid agencies - many of which are based there - have to operate with extreme caution, say correspondents.
Militants in Pakistan, and many Pakistanis, are angry at US involvement in the war in Afghanistan and US drone attacks which have left hundreds dead, mainly in the volatile border area between Pakistan and Afghanistan.