Iraqi officials say at least 250 people died in Tuesday's bombings in the north of the country - the deadliest attack on a single area since the 2003 war.
Members of the Yazidi sect had been targeted before the blasts
Four bombs devastated two villages of the Yazidi, a Kurdish religious sect, near the city of Mosul, and left at least 350 people injured.
Rescuers are continuing to dig through rubble to find survivors.
The US military blamed al-Qaeda for the bombs, and acknowledged that tackling insurgent attacks was a difficult task.
Brig Gen Kevin Bergner said the attack fitted the profile of "spectacular" strikes expected by al-Qaeda during the ongoing US "surge" operation.
The BBC's Richard Galpin, in Baghdad, says that with the Americans concentrating on their military effort in the capital, officials fear the insurgents are moving into new areas where they can attack so-called soft targets.
The US commander in Iraq, Gen David Petraeus, will issue a report into the progress of the surge in September.
In a statement, the White House insisted US forces and the Iraqi government would continue to "beat back" the "vicious and heartless murderers".
Iraq's Prime Minister Nouri Maliki and President Jalal Talabani, a Kurd, also added their condemnation.
The co-ordinated bombings in the villages of Qataniya and Adnaniya involved a fuel tanker and three cars, officials said.
KEY FACTS: THE YAZIDIS
Religious sect found in northern Iraq, Syria and the Caucasus
Number about 500,000 worldwide, but largest number in northern Iraq
Doctrine is an amalgam of pagan, Sabean, Shamanistic, Manichean, Zoroastrian, Jewish, Christian and Islamic elements
Yazidis believe in a Supreme God, but do not believe in evil, sin, hell or the devil
Violation of divine laws can be expiated by metempsychosis, or the transferring of a soul from one body to another
Principal divine figure, Malak Taus (Peacock Angel), is the supreme angel of the seven angels who ruled the universe after it was created by God
The force of the blasts ripped apart houses, many of which were made of no more than clay and mud.
"My friend and I were thrown high in the air. I still don't know what happened to him," Khadir Shamu, a 30-year-old Yazidi, told the Associated Press news agency.
Hospitals in the area are continuing to receive wounded with terrible injuries. A new bride in one hospital said her husband and nine of his family were killed.
A curfew has been clamped on the area and rescue work continues with many of the victims under rubble, while local hospitals continue to receive badly wounded survivors.
Potential blood donors were streaming into hospitals treating the injured in Nineveh and Dahuk provinces, AFP news agency said.
'Hands and shovels'
The mayor of Sinjal, another nearby town, said he expected the final death toll to rise.
"We are still digging with our hands and shovels because we can't use cranes as many of the houses were built of clay," Dhakil Qassim told AP.
"We are expecting to reach the final death toll tomorrow or [the] day after tomorrow as we are getting only pieces of bodies."
A spokesman for the Kurdistan regional government, a semi-autonomous authority which governs three northern Iraqi provinces, described the Yazidi as a "threatened minority" and said Kurdish forces might have protected them from harm.
"But because of the inaction of the government in Baghdad and their inability to protect the population they are suffering the way they are now," he added.
Tensions between the Yazidi sect and local Muslims have grown since a Yazidi girl was reportedly stoned by her community in April for converting to Islam.
The sect is due to vote later alongside other Kurds outside the Kurdish autonomous region in a referendum on joining the grouping. Correspondents say the planned referendum makes northern Iraq's Kurds a target for politically-motivated attacks.
Yazidis worship an archangel, sometimes represented by a peacock figure believed by some Christians and Muslims to be the devil.