The aid worker was shot by masked men carrying automatic rilfes
Gunmen in Pakistan have shot dead an American aid worker and his driver in the north-western city of Peshawar.
The men were killed just outside their office in the University Town area. It is not clear who the attackers were.
Violence has surged in the north-west in recent months with a wave of attacks blamed on Islamist militants.
A number of missile strikes inside Pakistan's tribal areas by US troops based in neighbouring Afghanistan have fuelled anti-American sentiment.
The BBC's Mark Dummett in Islamabad says that the security situation across Pakistan has steadily worsened over the past few years, with Taleban militants holding sway over a large stretch of North West Frontier Province (NWFP).
But our correspondent says attacks of this kind on foreigners in Pakistan are rare. Across the border in Afghanistan aid workers and other foreigners have increasingly been targeted in recent months.
Gunmen attacked the car of a US diplomat in Peshawar in August, but she survived unhurt.
Our correspondent says that it is more common for militants in Pakistan to launch suicide bomb attacks against military or government targets.
The US embassy has refused to identify the man until his next of kin are informed.
But an embassy spokesman confirmed that the victim was an American. He said the dead man was not a diplomat nor was he travelling on an official assignment.
US media reports identified the aid worker as Stephen Vance. Earlier reports had given his name as Stephen David.
Police say the American worked for a US-funded project to help develop the troubled tribal belt - a large swathe of which is now controlled by militants.
Eyewitnesses say the aid worker and his driver were shot by a group of masked gunmen as they drove to their office in University Town, a wealthy suburb of the main city in north-west Pakistan.
The attackers blocked the men's vehicle in a narrow street with their own car before opening fire with automatic weapons, officials said.
"Several bullets hit them, and they died in the vehicle," police official Arshad Khan told the Associated Press.
Meanwhile, two Pakistani security officers were killed and several others were injured in a suicide attack in north-western Pakistan, officials said.
Peshawar was hit by a suicide bombing on Tuesday
The suicide bomber rammed an explosives-laden car into a temporary security forces camp in Shabqadar area of Charsadda district, 25km north of Peshawar.
Areas close to Peshawar - the biggest city in north-west Pakistan - are known to be Taleban and al-Qaeda strongholds.
The region has been hit by several bombings and suicide attacks recently.
On Tuesday, a suicide bomber walked up to the gate of a stadium in Peshawar and blew himself up.
The attack happened as the governor of North West Frontier Province left after a sports tournament.
He was unhurt but at least one man was killed and three people were injured.