BAGHDAD, Iraq (CNN) -- A suicide car bomb detonated Sunday evening near a market in Balad, killing at least 25 people and wounding 40 others, an Interior Ministry official said.
U.S. Defense Secretary Robert Gates made an unannounced visit to Iraq on Sunday.
The bomb exploded near an Iraqi army checkpoint outside the market in Balad, north of the capital.
The bombing came as U.S. Defense Secretary Robert Gates arrived in Baghdad on an unannounced visit. He was in Baghdad to meet Iraqi government leaders and U.S. commanders, including Gen. David Petraeus, the top U.S. commander in Iraq.
It also came on the same day that insurgents killed five members of a neighborhood watch group that has been helping coalition forces fight al Qaeda in Iraq elements.
The Sunni fighters -- known as Sons of Iraq in U.S. military parlance -- were killed in Nineveh province, north of Baghdad, a U.S. military spokesman said.
Five more fighters were wounded when insurgents ambushed their compound near Sinjar early Sunday morning, Capt. Stephen Bomar said.
The fighters -- who also have been called popular committees, Concerned Local Citizens and Awakening Councils -- returned fire, killing 10 insurgents, Bomar said.
Insurgents also attacked a group of civilians in Nineveh province, killing two men and a child. The attack also left wounded six civilians, two infants among them, the U.S. military said.
The attackers were inside a truck and opened fire on a crowd near the village of Romo, Bomar said.
"This is an intolerable and disgusting act against the innocent people of Iraq that cannot be tolerated," said Maj. Gary Dangerfield, spokesman for the 3rd Armored Cavalry Regiment.
Earlier this month, an umbrella insurgent group led by al Qaeda in Iraq issued a warning that it was launching an offensive in Nineveh province. Islamic State of Iraq said its efforts are aimed at countering an operation by the Iraqi military to clear the area of Islamic fighters loyal to the militant network.
Al Qaeda in Iraq and other militants have long had a presence in Nineveh and its capital Mosul, a destination for insurgents sneaking into Iraq from Syria.
• A U.S. soldier was sentenced to 10 years in prison after being found guilty Sunday of the unpremeditated murder of an Iraqi citizen, the U.S. military said said. Sgt. Evan Vela was court-martialed for the May 11 murder, which occurred south of Baghdad. He also was found guilty of planting a gun on the body and for making a false official statement.
• The United Nations warned Sunday that its relief agencies working to feed Iraqi refugees may be unable to get food to those in need this spring because funds are quickly depleting. Despite an appeal for money last month, the U.N. World Food Program said in a statement that it is $113 million short of what is needed. E-mail to a friend