"At the time of the attack, a nearby unit heard explosions and attempted to establish communications, but without success," he said in a statement.
"Coalition forces arrived within an hour, secured the site, and immediately initiated a search. The names of the soldiers are being withheld pending final identification and notification of next of kin," he said.
"Make no mistake: We will never stop looking for our soldiers until their status is definitively determined, and we continue to pray for their safe return," he added, as helicopters and jets were scrambled to join the hunt.
Mahmudiyah lies 30 kilometres (20 miles) south of Baghdad in an area of farmland and palm groves known as an insurgent stronghold. Since the US invasion of March 2003 the region has been nicknamed the "Triangle of Death".
Earlier, US marines on patrol on a road west of Baghdad near the restive city of Fallujah told an AFP reporter travelling with them that they had been told the missing soldiers were thought to have been captured.
"We got word about 0900 (0400 GMT) that three soldiers were missing. Last we heard was that they were headed in this direction, presumed captured," said Gunnery Sergeant James Curtis of the US Marines 2nd Battalion 6th Regiment.
Curtis's patrol threw up a checkpoint on a stretch of road on the eastern edge of Fallujah in a bid to intercept the abductors if they attempted to bring their captives there from the area west of Baghdad.
Some of the trucks and cars arriving at the roadblock were marked as having been searched, showing that other military checkpoints had been set up on the road, which leads from Fallujah towards both Mahmudiyah and the capital.
"Checkpoints have been established throughout the area in a concerted effort to focus the search and prevent potential movement of missing soldiers out of the area," Caldwell's statement said.
"Coalition forces have engaged with local leaders to enlist their support in providing any information they can, and these engagements continue."
UPDATE II: The whack-jobs are out in force on this one, at least at one site ... read the comments here. UPDATE III: More details:
The military refused to specify whether the Iraqi interpreter was among those killed or among the missing or give more details about where the bodies were found.
An Iraqi army officer, who was familiar with the search but spoke on condition of anonymity because he was not authorized to disclose the information, said he saw five badly burned bodies inside a Humvee at the attack site, suggesting the remains may not have been recognizable.
He also said joint U.S.-Iraqi forces had sealed off the area and were conducting house-to-house searches in the area, rounding up dozens of suspects. The military declined to comment on detentions but said troops were looking for suspects.
The Iraqi officer said U.S. troops singled out seven suspects out of as many as 50, including a wounded man who was hiding in a house and confessed to participating. He said most of the houses searched near the attack contained only women and children because the men had fled after hearing news of the attack, fearing arrest.
mullah cimoc say him warning ameriki people in 2003 when bush the invading iraq.
mullah cimoc then tell all ameriki saying please to reading the books of him chairman mao tse tung on guerilla warfare strategies and tactical.
mullah cimoc say in 2003 him insurgent running the enrage the bear tactical.
bear so ferocious, but running to and fro and the lunging to and fro, finally getting tired and the weaker and finally after the tormenting after the exhaustion him wanting to be killed just for ending the suffering.
this usa now in iraq. so the weaken, now the guerilla more aggressive but still the so careful. the bear still roar but hearing now the weakness.
in this time now all muslim knowing that in new iraq only him who killing so many ameriki soldier having the status and the power.
the collaborator him to die and all the family too, unless so torture by ameriki.
only one kind of the voting to count in new iraq. this ballot him calling the body bag containing the ameirki soldier ballot. if not have the this ballot, not having him vote.
this new man in new iraq him true warrior face every day adversity. him only man with political power in new iraq.
for this reason now the killing for starting so much against ameriki soldier. the wife telling the husband, "Omar, you needing for killing three ameriki now so our children him going the college and have good job in new iraq". Also, "you not my husband if not killing ameriki soldier."
this new kind of gold rush, but this rush him calling this the rush for kill ameriki soldier.