Gunmen posing as security forces killed 21 people on a highway in southeastern Iran near the border with Afghanistan and Pakistan, the national chief of police said Friday.

Iranian police called the assailants "rebels." But there are no well-known political opposition groups operating in southeastern Iran, a region known for gangs of drug traffickers who have frequently clashed with security forces and occasionally kidnapped people.

National police chief Gen. Esmaeil Ahmadi Moghaddam accused U.S. and British intelligence of being behind the attack on Thursday night, the official Iranian news agency reported.

Iran made similar accusations in the past after bombings in the southwestern province of Khuzestan, where there has been unrest connected to the country's Arab minority. Blasts in the Khuzestan capital Ahvaz killed six people on Jan. 24. The United States and Britain denied any involvement.

In Thursday's violence, gunmen posing as policemen and soldiers stopped people and killed them on the Zabol-Zahedan road in Sistan-Baluchestan province, which borders Pakistan and Afghanistan, the agency quoted Moghaddam as saying.

"People thought they were Iranian police (ordering them to stop)," said Moghaddam, who flew to the provincial capital of Zahedan on Friday to visit the scene of the attack. He said 21 people were killed and seven injured.

Moghaddam said police had information indicating U.S. and British intelligence agents had met representatives of the "rebels," but he gave no details.

"It appears that a plan to create instability and religious hatred, similar to the bombing of the Shiite shrine in Samarra (in Iraq), is being pursued here," IRNA quoted Moghaddam as saying.

That shrine attack on Feb. 22 unleashed a wave of sectarian violence that left hundreds dead and intensified fears of all-out civil war in Iraq.

Moghaddam said police were searching for the attackers, who may have fled to Afghanistan.

There has been sporadic violence in recent months in the same region where Thursday's attacks occurred. In December, gunmen in Sistan-Baluchestan kidnapped nine Iranian soldiers. They freed seven soldiers in January, but the fate of the other two is not known.

The same month, Sistan-Baluchestan bandits killed a guard of Iranian President Mahmoud Ahmadinejad hours before he was due to visit the province.

In November, a clash between police and bandits in southeastern Iran resulted in the death of two gangsters and at least five policemen.