Iraqi security forces found 53 corpses, blindfolded and handcuffed, in a town south of Baghdad early on Wednesday, local officials said.
They said the bodies had been left in the mainly Shia Muslim village of Khamissiya, about 25 km southeast of the city of Hilla, near the main highway running from the capital to the southern provinces.
The head of the provincial council, local police and the governor's office all confirmed the discovery of the bodies, but had no immediate information on the identity of the dead, who appeared to have been killed execution style.
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He said the victims appeared to have been killed overnight after being brought by car to an area near the main highway running from Baghdad to the southern provinces, about 25 km southeast of the city of Hilla.
The identity and sectarian affiliation of the dead people was not immediately clear, he said.
Sunni militants have been carrying out attacks around the southern rim of Baghdad since spring. In response, Shia militias have been active in rural districts of Baghdad, abducting Sunnis they suspect of terrorism, many of whom later turn up dead.
The tit-for-tat attacks have escalated dramatically since Sunni Islamist fighters seized control of large parts of northern and western Iraq last month, sweeping towards Baghdad in the most serious challenge to the Shia-led government of Prime Minister Nouri al-Maliki since the withdrawal of U.S. forces in 2011.
Mass killings of scores of victims have become a regular occurrence in Iraq for the first time since the worst days of sectarian and ethnic cleansing in 2006-2007.
The Sunni insurgents, led by the group known as the Islamic State of Iraq and Syria, which considers all Shias heretics who must repent or die, boasted of killing hundreds of captive Shia army troops after capturing the city of Tikrit on June 12. They put footage on the internet of their fighters shooting prisoners.
In the following weeks more than 100 Sunni prisoners died in two mass killings while in government custody. The Shia-led government officially says they were killed in crossfires when their guards came under attack, first in a jail in Baquba north of Baghdad and then in a convoy moving prisoners from Hilla. Sunni leaders say the prisoners were executed by their guards.
Other suspected mass killings
Amnesty International and the United Nations have reported several other suspected incidents of mass killings of prisoners in government custody.
The fighting between ISIS, backed by other armed Sunni groups, and the army, backed by Shia militias, threatens to split the country.
The renewed sectarian war has brought violence to levels unseen since the very worst few months of the fighting that followed the U.S. invasion to topple Saddam Hussein in 2003.
Abductions have also increased. On Friday, 17 Sunni Muslims were taken from the Musayyib area and briefly held by security forces and Shi'ite militia, a local tribal leader said, while a prominent sheikh was also kidnapped by unidentified men.
Kurdish leadership lashes out at Maliki
Sunnis have backed the Islamic State's offensive because of the widespread view that they have been oppressed under the Shi'ite-led government of Prime Minister Nouri al-Maliki.
The United States and other countries have called for politicians to set up a more inclusive government in Baghdad following a parliamentary election in April. But the new legislature has so far failed to agree on leadership for the country, leaving Maliki in power as a caretaker.
Sunnis and Kurds demand he leave office, but he shows no sign of agreeing to step aside. The Kurds are now closer than ever to abandoning Iraq altogether, with Massoud Barzani, leader of their autonomous region, calling last week for his parliament to ready a referendum on independence.
Kurdish leader slams Maliki
In a statement late on Tuesday, Barzani launched a withering attack on Maliki, saying his eight years in office had brought disaster to Iraq and set the stage for its latest conflict.
"Today a dangerous precedent is being set, of feeding a chauvinistic campaign of ethnic hatred based on the distortion of reality... to serve political objectives and narrow partisan interests of the person who has caused Iraq to be led from failure to failure and crisis to crisis," Barzani said.
Kurdish forces have exploited the turmoil to seize control of the city of Kirkuk and its huge oil reserves a month ago, achieving a long-held dream. They regard the city, just outside their autonomous region, as their historical capital, while its oil could provide ample revenue for an independent state.
"We have said we are not prepared under any circumstances to accept for our will to be bent, and go back to square one and face what reminds us of the policies that drowned Kurdistan in seas of the blood of its civilians and turned their homeland to ruins and mass graves," Barzani said in his statement, referring to years of oppression under Saddam.
"That is what we have clearly faced throughout the period of abuse of power during the two disappointing terms of the prime minister."
Maliki hit back in a weekly address on Wednesday, accusing Kurds of allowing their provincial capital Arbil to become a haven for the Islamic State and other militants, including former members of Saddam's now-banned Baath Party.
"We will never be silent about Arbil becoming a base for the operations of the Islamic State and Baathists and al Qaeda and the terrorists," he said.
Many Sunni Muslims who fled the mostly Sunni northern city of Mosul during the militants' offensive have ended up in Arbil.