Mohammad Bakhsh Abbasi, the Pakistani ambassador to Iran, has been injured after being assaulted by an Afghan citizen in Tehran.

"The ambassador was attacked in his car and he was injured. The attacker, who was an Afghan citizen, has been arrested and is being interrogated now," Ramin Mehmanparast, an Iranian foreign ministry spokesman, said on Tuesday.

Mehmanparast said the ambassador was not accompanied by a driver or guards at the time of the incident.

Alireza Ronaghi, Al Jazeera's correspondent in Tehran, said that the 21-year-old Afghan national was armed with a knife, but this was not used in the attack.

"The ambassador fell on his arm and hit his head on the pavement. He has to be under supervision for 24 hours to confirm he has no head injuries," Ronaghi said.

There was no immediate word on the motive for the assault and the Pakistani embassy refused to comment.