Four U.S. soldiers conducting a public-health assessment were wounded Sunday morning when their Humvees were ambushed in downtown Baghdad (search), a U.S. military spokesman said.

The soldiers, in two Humvees (search), were stopped in midmorning traffic when an assailant approached and fired at them from a small-caliber weapon, said Capt. David Connolly, a military spokesman in Baghdad.

One of the soldiers' injuries were serious, he said without elaborating.

"There are still some small pockets of resistance in Baghdad," Connolly said.

He did not know whether the soldiers returned fire or whether the shooter was apprehended.

The soldiers, who were taken to a combat support hospital, were on a public health mission when the attack occurred, Connolly said. Their names were being withheld pending notification of family.