US forces have stepped deeper into the Iraqi government's fight to cripple Shi'ite militias, launching air strikes in the southern city of Basra and firing a Hellfire missile in the main Shi'ite stronghold in Baghdad.

The American support occurred as Iraqi troops struggled against strong resistance in Basra and retaliation elsewhere in Shi'ite areas - including more salvos of rockets or mortars into the Green Zone in Baghdad.

More than 180 people have been killed since the clashes broke out four days ago.

The southern city of Nasiriyah also saw fierce battles yesterday with local medical officials reporting 36 deaths.

US President George W. Bush, meanwhile, has sought to bolster Iraqi Prime Minister Nuri al-Maliki saying his government faces a defining moment in its deadly crackdown against Shi'ite militants.

Fighting that began this week in Basra has drawn US forces into the fray. Bush cast it not as a setback, but as a sign Maliki's government is willing to confront 'criminal elements or people who think they can live outside the law.'

'I would say that this is a defining moment in the history of a free Iraq. There have been other defining moments up to now but this is a defining moment,' the president said at a White House news conference. 'It is a necessary part of the development of a free society.'

The fighting has spread to Baghdad and pitted Iraqi security forces against Shi'ite fighters loyal to radical anti-US cleric Moqtada al-Sadr. On Friday, authorities shut down Baghdad with a strict curfew but attacks and clashes in the capital continued.

Bush spoke after the US military stepped in to support Iraqi security forces by launching air strikes in Basra for the first time and battling militants in Baghdad.

Like Bush, Iraqi authorities describe their adversaries as outlaws. But Sadr loyalists say Maliki's Shi'ite-led government is using military force to marginalise rivals before elections due by October.

Bush said the crackdown showed Iraqis that Maliki was willing to act against fellow Shi'ites. 'In order for this democracy to survive, they must have confidence in their government's ability to protect them and to be even-handed,' he said.

With about 10 months left in office and his approval ratings stuck near the lows of his presidency, Bush has made a series of speeches in recent weeks defending his Iraq policy and appealing for patience from the American public.