"The explosion was caused by contractors from the Khan Amoo company who are building a gas pipeline from Asaluyeh to Khuzestan," Ghasem Moradi, a governorate security official, was quoted as saying by the Iran daily.

"An investigation showed that no airplane attacked Iranian facilities," the paper reported him as saying.

Last Wednesday, a big blast in the region of Bushehr, where a controversial nuclear plant is being built with Russian help, sparked fears of attack when witnesses reportedly saw a missile being fired from an unidentified plane.

Iran newspaper said the contractor company had the necessary permit to carry out the controlled explosion on the border between the southern provinces of Khuzestan and Bushehr.

Following worldwide reports of the blast, Iran accused foreign media of contributing to a "psychological war being waged by the United States against Iran" and warned it would respond to any military strike.

"The government and especially the intelligence ministry believe that those officials who spread this news can be investigated by the security apparatus," government spokesman Abdollah Ramezanzadeh told reporters on Monday.

"It has to be known why and who was responsible for breaking this false news which has affected our economy and regional relations," he said, adding that the offenders and their motives must answer to the nation.