The bodies of at least 20 people who were shot dead have been found near the western Iraqi town of Qaim, police say.
The massive blast in Baghdad shook buildings in the area
One of the dead had an Iraqi police identity card and others may have been national guards, officials said.
In the capital, Baghdad, a suicide car bomb attack reportedly carried out by a group linked to al-Qaeda killed three and injured more than 20 people.
The bomber drove a rubbish truck near the ministry of agriculture and Sadeer hotel before detonating explosives.
The ministry building caught fire but police said the main target may have been the hotel, which is used by Iraqi police and Western contractors.
Militants led by Jordanian-born militant Abu Musab al-Zarqawi purportedly claimed responsibility for the blast in an internet statement.
In other incidents across the country:
- Iraqi Planning Minister Mehdi al-Hafidh survives an assassination attempt in Baghdad which leaves one of his bodyguards dead
- A militant group, the Islamic Army in Iraq, posts an internet video showing two Sudanese drivers who have been taken hostage for working with US forces
- A policeman is killed and three others wounded when their patrol is hit by a roadside bomb in the southern city of Basra
- The director of al-Furat hospital in Baghdad, Dr Adil Abd-al-Karim, is shot dead on his way to work
- Gunmen open fire in eastern Baghdad on a minibus carrying workers for a Kuwaiti company, killing one and wounding three
The BBC's Jim Muir in Baghdad says the feeling in Iraq is that the tempo of insurgent attacks is rising again after a lull during February in the wake of elections.
The political process in Iraq is currently stagnant during efforts to form a government, our correspondent adds, allowing insurgents the opportunity to renew attacks.
Shot in the head
The 20 bodies, all in civilian clothes, were discovered on Tuesday night in a field by a village near the border with Syria.
They had been shot at close range in head or chest, and may have been dead for as long as a week, reports said.
One corpse was reported to have been carrying a police identity card.
The incident came just a day after police reported the discovery of 15 decapitated bodies on a disused army base south of Baghdad, some of them women and children.
Officials said the culprits had been arrested and that they were criminals rather than insurgents.
Pall of smoke
The militants who claimed the Baghdad suicide bomb attack said it had been timed to avoid hurting Muslim passers-by.
It began at 0630 (0330 GMT) with insurgents firing a rocket-propelled grenade on to a checkpoint sealing off the main road to the ministry and Hotel Sadeer.
The Baghdad attack created a plume of acrid black smoke
"I saw a black BMW pull up to the checkpoint and shoot one guard. There was a change of guard at the time. The BMW killed the guard and cleared the way for the garbage truck to enter," witness Haidar Hamid told the AFP news agency.
Another witness said two men in police uniforms had subsequently shot another guard at a checkpoint to the main entrance to the ministry.
Guards at the hotel started to fire heavily down the street, Mohammed Fadl said.
The suicide bomber then drove the truck though the checkpoints and exploded it.
The massive blast covered the sky with a pall of black smoke - the result of the wreckage of the truck and about 20 other vehicles in the parking lot which were damaged.