BAGHDAD — Australian troops ended their main combat mission in Iraq yesterday, handing over their responsibilities in southern Iraq to American forces.

An estimated 550 Australian troops, who served in a training and back-up role to Iraqi forces in the provinces of Dhi Qar and Muthanna, made the transfer in a ceremony at Camp Talil outside Nasiriya, Captain Chris Ford, a British military spokesman in southern Iraq, said.

Meanwhile, American officials announced that a bomb killed an American soldier in Baghdad yesterday. The military said the blast was caused by an explosively formed penetrator, an armor piercing bomb that the U.S. Army associates with hard-line Shiite Muslim militants and believes are supplied in part by Iran. It was the first American death this month. In May, the U.S. military said 19 soldiers were killed, the lowest monthly total since the American-led invasion of Iraq in March 2003. At least 4,085 U.S. military personnel have died in Iraq since the war began.

The U.S. military says violence in Iraq has dropped to its lowest levels since March 2004. The current calm relies on fragile arrangements: a deal struck by the American military with former Sunni insurgents to pay them to guard their neighborhoods or regions, as well as the decision by the Shiite cleric Moqtada al-Sadr to freeze his powerful militia’s armed activities. In March and April, violence soared as Mr. Sadr’s militia clashed with American and Iraqi forces. The bloodshed dropped once more after Mr. Sadr commanded his followers again to lay down their weapons.

The Australian flag was lowered yesterday at the ceremony in Dhi Qar province. Ahead of his election in November, Prime Minister Rudd had pledged to end his country’s front-line military role in Iraq.

Australia has said it will keep several hundred troops in Iraq to guard its diplomats and act as liaisons on security and headquarter functions. It also intends to commit two maritime surveillance aircraft and a warship to help guard oil platforms.

Australia follows in the footsteps of other American allies who have ended or drastically curtailed their work in Iraq amid domestic discontent over the war. Poland has announced its intention to end its military presence in Iraq by the end of the year. Spain became the first of America’s Western allies to withdraw its forces in 2004.

British troops in Basra province serve in a backup role at the request of Iraqi forces. British numbers have dropped to 4,000 troops from 18,000 in May 2003. Italy, which once stationed combat troops, now serves in a training capacity to the Iraqi national police.