Gunmen stormed an apartment building in a Baghdad neighborhood Saturday, killing at least 29 people, mostly women.
Iraqi police report finding bodies scattered throughout the building and blood streaming down the stairs.
No one has claimed responsibility for the attack and the motive is also unknown.
Report: Iraqi forces execute 255
Earlier Saturday, Human Rights Watch accused Iraqi security forces and government-backed militias of illegally executing at least 255 prisoners in the last month.
The New York-based rights group said the executions took place in six Iraqi towns and villages since June 9, calling them an "outrageous violation of international law."
The group said most of the victims were Sunni prisoners who were fleeing the Islamic State of Iraq and the Levant and other armed groups. At least eight of the murdered prisoners were less than 18 years old.
Human Rights Watch gathered statements from witnesses, security forces and government officials for the report.
The accusations come as violence continues to spread across Iraq, which is sending about 4,000 volunteers to the embattled city of Ramadi, west of Baghdad, to support government forces battling Sunni militants.
On Friday, a car bomb in northern Iraq's Kurdish-controlled city of Kirkuk killed at least 30 people. The dead include civilians and Kurdish soldiers manning a checkpoint.
Earlier, Kurdish forces seized two northern Iraqi oil fields near Kirkuk, saying they want to secure the facilities.
The central government in Baghdad has condemned the seizure and demanded the Kurds immediately withdraw.
Kurdish forces took control of Kirkuk and other northern areas nearly a month ago.
And in a further split between Kurds and the government, Kurdish politicians formally suspended their participation in Prime Minister Nouri al-Maliki's Cabinet, calling him "hysterical." The prime minister has accused Kurds of harboring Sunni extremists who have taken over much of northern Iraq and threaten Baghdad.
The Kurds have an autonomous zone in northern Iraq, and many have an eye on independence.
Nickolay Mladenov, United Nations special envoy to Iraq, is urging all Iraqi lawmakers to attend Sunday's session of parliament. He says failing to make progress on forming a new government could plunge Iraq into even more chaos.
Volunteers to Ramadi
Iraqi government TV reported that the 4,000 volunteers were being airlifted to Ramadi from the country's mostly Shi'ite regions of Karbala, Baghdad, Najaf and Basrah.
It said Anbar province governor Ahmed Khalaf al-Dulaimi made the announcement in a statement Saturday.
Some information for this report comes from AP, AFP and Reuters.