The bodies of more than 40 Iraqi army recruits have been found following an ambush near the Iranian border in the north-east of the country.
The bodies of the dead men were found by the roadside
Police said many of the soldiers had been shot in the head.
Hours later, a US diplomat was killed in a mortar attack at Camp Victory near Baghdad airport.
Ed Seitz, assistant regional security officer at the US embassy in Baghdad, is believed to be the first US diplomat killed during the current war.
A website often used by militants published a statement attributed to the group led by Jordanian militant Abu Musab al-Zarqawi in which it claimed to have carried out the army recruits massacre.
Police said the bodies of the recruits - who were not armed - were found near the village of Mandali, south of Baquba.
"This was an execution. We found the dead lying face down by the roadside with a single bullet wound to the head," Iraqi national guard commander Ali al-Kaaki is quoted as saying by the AFP news agency.
A local police official said that all had their hands crossed behind their heads.
Interior Ministry spokesman Adnan Abdul Rahman said 37 bodies were found shot dead on the side of the road, while the bodies of 12 others were found in a burned bus.
There was confusion over the precise figures. The Iraqi National Guard said 48 troops and three drivers were killed.
In other incidents:
The BBC's Claire Marshall in Baghdad says the army recruits were believed to have just finished a training course at a camp near the Iranian border.
They were ambushed on a remote road in Diyala province at sunset on Saturday.
They were travelling in three minibuses, in civilian clothes.
According to some reports, they were stopped at a fake checkpoint and militants fired rocket-propelled grenades at the vehicles.
"It appears that they were ambushed by a large, well-organised force with good intelligence," a senior security official told the Reuters news agency.
"There was probably collusion among the soldiers or other groups," Diyala province deputy Governor Aqil Hamid al-Adili told al-Arabiya television, the AP news agency reported.
"Otherwise, the gunmen would not have gotten the information about the soldiers' departure from their training camp and that they were unarmed," he added.
Reports suggest that the recruits were mainly from Shia cities in the south of Iraq, including Basra, Amara, Kut and Nasiriya, and were returning there after training in the desert camp.
Correspondents say Iraq's fledgling security forces are a prime target for militant groups.
On Saturday, 20 Iraqi policemen were killed and 47 wounded in two separate suicide car bombing attacks.