With the overall U.S. military death toll in Iraq nearing 4,000, the latest killings mark a significant rise in deadly attacks against Americans.
At least 3,987 members of the U.S. military have died since the beginning of the Iraq war in March 2003, according to an AP count. The figure includes eight military civilians.
Navy Lt. Patrick Evans, a military spokesman, told The Associated Press that three soldiers were killed Wednesday in a rocket attack on Combat Outpost Adder near Nasiriyah, about 200 miles southeast of Baghdad. Two other soldiers were wounded.
The attack came a day after an American soldier died when a roadside bomb hit his patrol near Diwaniyah, 80 miles south of Baghdad.
Eight soldiers were killed in a pair of bomb attacks on Monday, the heaviest single day of U.S. casualties since September.
Three of those soldiers died in a roadside bombing in Diyala, a violent province where al Qaeda in Iraq has been active.
The five others were killed while on foot patrol in central Baghdad. A suicide bomber approached them and detonated his explosives vest. Three Americans and an Iraqi interpreter were wounded.
The latest deaths came as a group of Iraqi tribal leaders, former politicians and intellectuals appealed Wednesday to the United Nations to take control of Iraq in a move they say would help U.S. troops leave the beleaguered country.
Both the Bush administration and the Baghdad government are unlikely to endorse the request, which was addressed to U.N. Secretary-General Ban Ki-moon and delivered to the Cairo offices of the organization.
"We believe that the only opportunity left for Iraq to be saved from a dark, but not inevitable future, is to engage the international community represented by the United Nations," the letter said. "Such a step will allow the American troops to leave and the occupation to be brought to its end."
The group's coordinators include Adeeb al-Jadir, Ahmed Al-Haboubi and Nouri Abdel Razak Hussein, politicians overthrown in 1968 when Saddam Hussein's Baath party came to power and long part of the liberal anti-regime opposition prior to the U.S.-led invasion in 2003.
The U.N. dramatically curtailed its operations in Iraq after an August 2003 suicide attack killed its representative and scores of others. The United States has been pushing for an expanded U.N. role in Iraq but that did not include supervising the country.
The Iraqi group said the world body should supervise a new security plan to restore order during a transitional period and prepare for new elections of a government to replace Nouri al-Maliki's troubled cabinet.
Representatives for the campaign will travel to the U.N. headquarters in New York to seek support from key members, said al-Haboubi, a former government minister.
"We are also ready to discuss our proposals with U.S. officials," he said.
The men said the petition was signed by dozens of Iraqi dignitaries and they had scores of supporters in Iraq who preferred to remain anonymous for now to avoid harassment.
On Wednesday, the U.S. military acknowledged that a roadside bomb targeting a passing U.S. convoy had struck near a passenger bus, a day after initially claiming no one died in the attack.
U.S. military spokesman Maj. Gen. Kevin Bergner could not confirm the number of casualties, but said no U.S. forces were involved in any gunfire that followed.
"We are still working with Iraqi security forces, and those now investigating the detailed circumstances of that attack, to learn whatever else we can," Bergner said.
Dr. Hadi Badr al-Riyahi, head of the Nasiriyah provincial health directorate, confirmed that the attack on the bus traveling from Najaf to Basra killed 16 civilians and wounded 20 about 50 miles south of Nasiriyah.
At the time of the attack, a local policeman and the assistant bus driver also said 16 people were killed on the bus, which was riddled with holes that appeared to be caused by shrapnel or bullets.
On Tuesday, violence reportedly killed a total of at least 42 people across Iraq. The sudden spike comes in the wake of a 60 percent drop in attacks across the country since June, according to U.S. military figures.
According to an AP count, at the height of unrest from November 2006 to August 2007, on average approximately 65 Iraqis died each day as a result of violence. As conditions improved, the daily death toll steadily declined. It reached its lowest point in more than two years on January 2008, when on average 20 Iraqis died each day.
Those numbers have since jumped. In February, approximately 26 Iraqis died each day as a result of violence, and so far in March, that number is up to 39 daily. These figures reflect the months in which people were found, and not necessarily - in the case of mass graves - the months in which they were killed.
In other developments:
The U.S. military did not immediately respond to requests for comment.
The fighting comes two weeks after Turkey's eight-day incursion into northern Iraq to flush out the rebels of the Kurdistan Workers' Party, who have been battling the Turkish government since