Iraqi forces backed by US troops launched a new operation against Shiite fighters in the south of the country early on Thursday after the expiry of a four-day deadline to surrender arms. "The operation started overnight. The situation is normal and there has been no trouble," said Colonel Mehdi al-Asadi, police spokesman in Maysan province where the crackdown against the militiamen has been launched. Asadi said the details of the operation would be announced at a press conference soon. Iraqi Prime Minister Nuri al-Maliki had given a four-day deadline to Shiite militiamen in Maysan and in its capital Amara to lay down their arms which expired on Wednesday.