A NEW offensive against Iraq's Mahdi Army - a militia blamed for sectarian death squad killings - is to be launched by the United States and Britain, it was claimed yesterday. US-led forces are to target the organisation, led by the Shiite cleric Muqtada al-Sadr, after the Pentagon described it as the biggest threat to Iraq's security. Diplomats say Washington is impatient to confront its key figures. Several officials in the Shiite political parties that dominate the unity government of the prime minister, Nuri al- Maliki, are also losing patience with Sadr's supporters and predict more raids like last week's...