Rioting erupted in Kabul Friday when scores of Afghan men set fire to two US embassy vehicles after one collided with a civilian car killing a number of occupants, officials and witnesses said.
Television pictures showed the vehicles in flames and young Afghan men throwing stones at them.
NATO's International Security Assistance Force (ISAF) said it had despatched a quick reaction force to the area, outside the American embassy and near US and Afghan army bases in the centre of the city.
An ISAF official said the vehicles involved belonged to the US embassy.
"We don't know yet how many people were killed in the accident," interior ministry spokesman Zemarai Bashery said.
Witnesses said four passengers of the civilian car died when it was hit by one of two armoured vehicles moving in convoy.
Police fired shots in the air to quell the violence, an AFP reporter witnessed.
Afghan security forces cordoned off the area, closing the road to Kabul's international airport, he said.
Local resident Saleh Ahmed said the accident happened when the civilian vehicle attempted to drive onto the main road from a side street and was hit by one of the two armoured vehicles.
"The civilian vehicle was trying to get into the main road when the two foreign vehicles hit it and killed all four occupants," he said. "People gathered around the crash site to see what had happened, got angry and started attacking the foreigners."
The AFP reporter on the scene said police helped the foreigners leave as the riot continued for about an hour before people started to disperse.
Young Afghan men threw stones and shouted "death to foreigners" and "death to Karzai," referring to President Hamid Karzai, he said.
A similar traffic incident led to massive riots that shook the capital in May 2006, leaving at least 14 people dead.
Deployments by the United States and NATO are nearing their peak of 150,000, concentrated in the southern provinces of Helmand and Kandahar, where a nearly nine-year Taliban insurgency is at its most intense.
A motorcycle bomb targeting a candidate in upcoming parliamentary elections killed a woman and a child in the southern city of Kandahar on Friday, police said.
The explosives-laden motorcycle was parked in a city centre alley used by the candidate, and detonated minutes after he passed by, provincial deputy police chief Fazel Ahmad Shairzad told AFP.
He blamed the attack on "enemies of Afghanistan," a term often used to refer to the Taliban.
The parliamentary election was originally scheduled for May but postponed until September 18.
Candidates appear to be the latest targets of the Taliban, who have stepped up a campaign of roadside bombs, suicide attacks and assassinations in recent months.
Candidate Sayedullah Sayed died after a mosque in the southeastern province of Khost was bombed last Friday as he was campaigning, injuring 20 people.
Also in the south of the country, NATO said that three foreign soldiers had been killed in two separate Taliban-style bomb attacks on Thursday. An ISAF spokeswoman confirmed all three were American.
A total of 408 foreign troops have died in the Afghan war so far this year, according to an AFP tally based on that kept by the icasualties.org website.
The toll for July is 86, compared with 102 in June, the worst month for foreign military casualties since the end of 2001.