Gunmen killed 22 Muslim viitors on their way to Syria as they were passing through a predominantly Iraqi province from the holy shrine city of Karbala.

Gunmen killed 22 Muslim visitors on their way to Syria as they were passing through a predominantly Iraqi province from the holy shrine of Imam Hussein in the city of Karbala, officials said on Tuesday.

The group had all been passengers on a bus passing through Anbar province, when their vehicle was stopped by gunmen at 9:30 pm local time 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 the visitors bound for Syria. On its way to Iraq's western neighbor, 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."

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 kilometers west of Karbala, which lies south of Baghdad.

Since the US-led invasion of 2003, the mainly 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.

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.