(CNN) -- Acts of terror and sectarian violence killed 659 Iraqis and injured almost 1,400 in November, according to casualty figures released by the United Nations Assistance Mission for Iraq.
Bombings account for many of the deaths, but U.N. officials warn that they're seeing a surge in execution-style killings that "have been carried out in a particularly horrendous and unspeakable manner," said Nickolay Mladenov, the special representative of the U.N. secretary-general for Iraq.
Two groups of execution-style killings were discovered Wednesday when Iraqi security forces found 13 unidentified bodies -- each shot in the head.
Eight of the bodies were found in the Arab Jabour district, a Sunni enclave in southern Baghdad. Five other bodies turned up in al-Shulaa, a Shiite area in the northwestern part of the capital.
"The Iraqi authorities should take immediate steps to find and hold accountable the perpetrators of these crimes and to implement effective measures to ensure the protection of all citizens," Mladenov said in a statement.
So far this year, 7,157 civilians and 952 Iraqi security forces have died in sectarian violence, according to U.N. figures.
CNN's Mohammed Tawfeeq contributed to this report.