BAGHDAD- A roadside bomb hit a bus traveling in southern Iraq on Tuesday, killing at least 16 civilians, while gunmen opened fire on another bus in the capital, leaving one person dead, police said.

The U.S. military, meanwhile, reported three American soldiers killed Monday by a roadside bomb north of Baghdad, bringing to eight the number of troops who died that day. It was the deadliest day for American forces in Iraq since Sept. 10, when 10 troops died.

At least 22 others were wounded in the attack on the bus traveling from Najaf to Basra, a policeman said on condition of anonymity because he wasn’t authorized to release the information. Five women and three children were among those killed.

Gunmen also sprayed another bus with machine gunfire shortly after it hit a roadside bomb in eastern Baghdad. One person was killed and four others were wounded, police said. The bomb was apparently targeting a nearby police patrol.

The U.S. military also said an interpreter was killed Monday along with three soldiers when they were hit by the bomb in eastern Diyala province. Another soldier was wounded.

That same day, five U.S. soldiers on a foot patrol were killed in central Baghdad when a suicide bomber approached them and detonated his explosives vest. Three American troops and an Iraqi interpreter were wounded. Iraqi police said two civilians also were killed in the attack.

The bombing showed the insurgents’ ability to strike in the heart of the heavily fortified capital. It was the deadliest single attack against the U.S. military since Jan. 28 when five soldiers were killed in a roadside bomb in the northern city of Mosul.

The suicide bomber hit the soldiers after they had left their Humvees and were chatting with shop owners, an Iraqi police officer who witnessed the attack said on condition of anonymity because he wasn’t authorized to speak to the media.

As part of the military’s counterinsurgency plans, U.S. bases are now inside neighborhoods and more U.S. soldiers are getting out of their armored vehicles to patrol Baghdad on foot.

While the face-to-face contact builds goodwill, it also gives suicide bombers, who often slip past security vehicle checkpoints by walking, better access to soldiers.

According to military figures, attacks in Baghdad are down 75 percent since June 2007, largely because of a boost in U.S. troops, a cease-fire by the radical Shiite cleric Muqtada al-Sadr’s Mahdi Army militia and the role of former Sunni militants and tribal groups who have switched sides to join U.S. forces against al-Qaida in Iraq.

But some fear that violence in Baghdad and elsewhere will accelerate after the withdrawal of thousands of American troops.

The drawdown began last December with the departure of one brigade, numbering about 5,000 troops, dropping the overall U.S. troop level in Iraq to 158,000. More troops are set to leave by July, though it has yet to be decided is whether further reductions will be made after that.

Also Tuesday, five people were killed when a suicide bomber in a truck laden with explosives detonated at a checkpoint near the headquarters of the Awakening Council in Duluiyah, 45 miles north of Baghdad, local police said. Another 13 people were wounded in the blast.

Awakening councils are made up of Sunni fighters who have accepted U.S. backing to switch allegiances and fight al-Qaida in Iraq.

And in Kut, 100 miles southeast of Baghdad, police arrested 30 people after clashes with the Mahdi Army left eight dead and another 14 people injured.

Associated Press writers Sameer N. Yacoub and Hamid Ahmed contributed to this report.