TAL AFAR, Iraq, April 20 (Reuters) - Authorities in the volatile Iraqi town of Tal Afar have imposed an indefinite curfew after militants distributed leaflets threatening to carry out chemical attacks, local officials said on Friday. Dozens of families have fled the religiously mixed town in northwestern Iraq in recent days after militants urged Sunni Muslim residents to leave the area. Suspected Sunni al Qaeda militants killed 152 people with a truck bomb in Tal Afar last month -- the deadliest single insurgent attack in Iraq since the U.S.-led invasion in 2003. That attack sparked reprisal killings by Shi'ite gunmen and Iraqi police in a Sunni district that left 47 dead. Most of Tal Afar's residents are Shi'ite and Sunni ethnic Turkmen. "We have imposed a total curfew from April 19th ... to calm people because these statements are not serious. We do not think that these groups have the capability to launch attacks using chemical weapons," said Najim al Jibouri, Tal Afar's mayor. "These groups only want to scare people," he said. Insurgents across Iraq have recently turned to car and truck bombs that spew out poisonous chlorine gas. When an explosion turns chlorine from solid or liquid form into gas, it causes severe burns when inhaled and can be lethal. Lieutenant Colonel Ali Hadi said most of the families who have fled Tal Afar are Sunnis. Those who have left since the reprisal killings last month have taken shelter in a camp set up for refugees in the nearby city of Mosul.
