A pick-up truck has crashed into a compound wall of the British embassy in Tehran, killing the driver.
Riot police held back anti-war protesters
The vehicle, which was carrying two 50-gallon drums of petrol, instantly exploded in flames, according to witnesses.
No injuries to embassy staff were reported, and the wall was said to have suffered slight damaged only.
A Foreign Office spokesman confirmed the driver of the vehicle had died during the incident.
"It is not clear whether it was an attack or an accident," he said.
The embassy is reviewing its security arrangements and is in discussion with the Iranian authorities, the spokesman added.
I don't think we should assume that every death, every crash, every incident everywhere suddenly threatens the action that we are taking
BBC correspondent Miranda Eeles in Tehran, said:
"The police are saying nothing now. Initially they did say it looked like it was an accident.
"But with the discovery of these two 50-gallon drums of diesel fuel in the back of the pick-up truck, I'm sure the investigation will probably turn its course."
British Home Secretary David Blunkett said there was no evidence the incident had been a terrorist attack.
Asked about it during an interview for BBC2's Newsnight, he said: "I may be proved wrong but I don't think we should assume that every death, every crash, every incident everywhere suddenly threatens the action that we are taking."
The crash happened at 2215 Iranian time (1845 GMT), close to the embassy's main gates.
"I doubt that this was a deliberate attack," Iranian government spokesman Abdullah Ramazanzadeh said.
The police cordoned off the area around the
embassy and an hour after the crash, a crane removed the wreckage.
The driver, who has not been named, was a
35-year-old married government employee, according to Ali Taala, general director of security and political affairs at Tehran's governor's office.
The incident came after tens of thousands of people marched through the centre of Tehran on Friday in protest at the US-led invasion of Iraq.
More than a dozen windows on the embassy's main office building were smashed by stone throwers as a crowd of more than 300 protesters chanted anti-war slogans outside the embassy.
Like most diplomatic missions in Tehran, the British embassy has not taken special security measures or evacuated staff due to the war in Iraq, judging the risk in Iran to be low.