At least 22 people have been killed by gunmen near Iran's border with Afghanistan, official reports say.
Many of those killed were government employees and the governor of the provincial capital, Zahedan, was seriously injured in the attack.
Police Commander Gen Esmail Ahmadi- Moqaddam said the gunmen posed as police and closed the Zabol to Zahedan road in Sistan-Baluchistan Province.
Gen Ahmadi-Moqaddam said US and British intelligence were behind the attack.
Correspondents say the Iran-Afghanistan border area is notorious for drug trafficking and kidnappings.
But it is not clear whether those responsible have political aims.
Speaking to reporters shortly after landing at Zahedan's airport on Friday morning, Gen Ahmadi-Moqaddam said the gunmen had closed the Zabol-Zahedan road at around 2100 (1730 GMT) on Thursday night.
"People thought they were Iranian police," he said.
After killing the civilians the attacks had fled across the border to Afghanistan, he added.
Gen Ahmadi-Moqaddam said he had information indicating that US and UK intelligence services had held meetings with the gunmen.
"The said intelligence services had instructed the local bandits on ways of undermining security in the region," he said.
"It seems that they are pursuing the same policy that they did in the Iraqi town of Samarra, that is, to provoke fighting between Shias and Sunnis."
The US and UK have troops stationed in southern Afghanistan as part of a Nato peacekeeping force.