BAGHDAD, Iraq (CNN) -- Five U.S. soldiers were killed when a "very large" roadside bomb exploded near their combat patrol in southern Baghdad, a U.S. general said Friday.
A combination of small-arms fire and rocket-propelled grenades followed the blast Thursday, said Army Maj. Gen. Joseph F. Fil Jr., who described the attack as "showing a level of sophistication that we have not often seen so far" during the period of the so-called "surge" in U.S. forces.
Fil said four soldiers were killed initially. Seven others were wounded, he said, and one of those later died.
The intensity of the attack, Fil said, goes to successes that U.S. and Iraqi forces have been making in clearing and holding Baghdad neighborhoods.
Attackers "are running out of maneuver space, and they are starting to fight very hard, and that's what we saw yesterday."
The attack puts the number of U.S. troops killed in Iraq this month at 99, part of the deadliest three-month stretch since the war began -- 104 in April and 126 in May.
Overall Iraq war deaths for U.S. military personnel total 3,569. Seven civilian contractors working with the Pentagon also have been killed.
"Occasionally there's a spike" in overall casualty figures, Fil said. "But overall they've come down.
"They have not come down with our own forces here in Baghdad," he added.
A U.S. military air attack on insurgents last week in Diyala province is under investigation by American and Iraqi authorities after claims surfaced that the assault killed villagers who were defending their community.
The incident occurred June 22 in a village southwest of Khalis near Baquba, the U.S. military said.
"Iraqi police were conducting security operations" when helicopters and ground forces "observed more than 15 armed men attempting to circumvent the [Iraqi police] and infiltrate the village," the military said at the time.
The military said "the attack helicopters, armed with missiles, engaged and killed 17 al Qaeda gunmen and destroyed the vehicle they were using."
An Iraqi Interior Ministry official said that authorities received complaints that some of those killed were civilians.
Investigators sent to the site by the ministry heard from witnesses that some of the dead were civilians and others were militants, the official said.
The U.S. Army also received reports from people saying that some of those killed were villagers defending their town, said spokesman Lt. Col. Christopher C. Garver.
Garver said an inquiry had begun looking into "the accounts of the Iraqi police, the villagers and all the other circumstances related to the incident."
The attack helicopters were from the 25th Combat Aviation Brigade and ground forces were from 3rd Brigade Combat Team, 1st Cavalry Division.
Anti-American Shiite cleric Muqtada al-Sadr announced Friday the postponement of a nationwide march to protest the recent bombing of the revered Al-Askariya Mosque in Samarra, according to Iraqi state TV.
Iraqi Prime Minister Nuri al-Maliki's office on Thursday indicated it would be better to postpone the July 5 march until after the road from Baghdad to Samarra is properly secured.
State TV reported that al-Sadr postponed the march to the mosque until further notice.
Al-Sadr had called on Shiite followers to march to the mosque, where twin minaret prayer towers were bombed June 13. Sunni militants are suspected in the attack, which followed a 2006 bombing at the shrine that sparked the current wave of nationwide Shiite-Sunni violence.
Al-Maliki said providing safety on the road between Baghdad and Samarra and "cleansing it of terror organizations will take a bit of time."
The Association of Muslim Scholars, an influential Iraqi Sunni group, warned that the march could spur violence. There have been threats from Sunni insurgent groups intent on targeting pilgrims planning to travel to Samarra. E-mail to a friend
CNN's Mohammed Tawfeeq and Saad Abedine contributed to this report.