Gunmen killed 22 Shiite Muslim pilgrims on their way to Syria as they were passing through a predominantly Sunni Iraqi province from the shrine city of Karbala, officials said on Tuesday.
The group had all been passengers on a bus passing through Anbar province, long a stronghold of Sunni insurgents and Al-Qaeda's front group in Iraq, when their vehicle was stopped by gunmen at 9:30 pm (1830 GMT) on Monday.
Separate violence in and around Baghdad, meanwhile, killed four people on Tuesday and wounded eight, an interior ministry official said.
"Gunmen dressed in military and police uniforms set up a fake checkpoint, made the passengers get off the bus, separated the men from the women and children before killing the men and fleeing," a Karbala official said, speaking on condition of anonymity.
"All of the bodies are now in the Karbala mortuary," he added.
A mortuary official, who also declined to be named, confirmed the toll.
The bus was one of the daily services departing from Karbala carrying Shiite pilgrims bound for Syria. On its way to Iraq's western neighbour, it must pass through the desert region of Anbar.
"Men in army uniforms boarded the bus and told us they had been attacked, and they asked us to hand over our mobile phones, which we did," said Umm Thair, a survivor who claimed her husband's body.
"They then asked, again very politely, for the men to get off the bus to be searched, and they complied. Then we heard gunshots and a few women who had gotten off the bus began screaming when they saw the bloodied bodies."
Signup to our newsletter and follow us on Facebook and Twitter!
At that point, two trucks burst through the fake checkpoint without stopping, she said.
"The terrorists understood that the truck drivers would alert the security services to what was going on, and they fled. A short time later, the army and the police arrived."
Earlier, police General Haider Rzayj said the gunmen had stopped the bus, which he said was coming from Syria, and killed the men before laying their bodies on the ground.
The attack took place in Nukhaib, about 200 kilometres (125 miles) west of Karbala, which lies south of Baghdad.
Since the US-led invasion of 2003, the mainly Sunni province of Anbar has been a stronghold of Al-Qaeda, whose members have killed numerous Iraqis and foreigners travelling the roads to Jordan and Syria.
While tribal militias have cracked down on the insurgents since 2007, they have not completely eliminated them.
Elsewhere in Iraq, two anti-Qaeda militiamen and two policemen were killed in a suicide bombing in the town of Tarmiyah, just north of the capital, and a magnetic "sticky bomb" explosion in al-Amriyah, west Baghdad, according to an interior ministry official who declined to be named.
Violence is down across Iraq from its peak in 2006 and 2007, but attacks remain common. A total of 239 people were killed in violence in the country in August, according to official figures.
A total of 1,860 Iraqis have been killed since the beginning of the year, according to an AFP tally based on government figures.