TEHRAN - Iranian security forces killed three people on Monday and arrested four others they suspected of being behind a pipeline blast which halted gas exports to Turkey last week, Iran's student news agency ISNA reported.Officials in Maku, a city in Iran's West Azerbaijan province, said the militants had crossed the border from Iraq to blow up the pipeline and that at least one was a Turkish citizen."'Jamil', a Turkish national and head of this terrorist group, was killed along with two others," Maku governor, Hamid Ahmadian was quoted as saying.Gas exports to Turkey were briefly halted due to a blast on Friday. The pipeline was swiftly repaired and gas flows were resumed on Saturday.