* Post-Ramadan feast time filled with apprehension
* Sectarian warfare part of Iraqi daily life again
* Iraqis grow frustrated with Islamic State militants
By Michael Georgy
BAGHDAD, July 28 (Reuters) - Iraqi police on Monday found the corpses of 15 people, including three women shot in the head in militia-style killings, a bloody start to the holiday marking the end of the Muslim fasting month of Ramadan, security sources said.
Fears have been growing of a relapse to the dark days of sectarian civil war which peaked in 2006-2007 since Sunni militants seized large swathes of the north last month, building on gains by comrades made in the west of Iraq.
Iraq's U.S.-trained and funded army unravelled in the face of the lighting advance, and Shi'ite militias now rival government forces in their ability to confront the group formerly known as the Islamic State in Iraq and the Levant.
Baghdad's morgues are filling up once again with victims of sectarian slayings, kidnappings are on the rise and the bloodshed is forcing families to flee abroad or move to neighbourhoods where they feel less threatened.
This year's Eid al-Fitr festival marking the end of Ramadan is filled with uncertainty and apprehension as Sunni insurgents set their sights on Baghdad and Iraqi politicians struggle to form a power-sharing government capable of tackling them.
Police found 15 corpses in different parts of the capital, security sources said. Among them were three women aged 25-30 who had been handcuffed and shot in the head execution-style in an industrial area just north of the Shi'ite Sadr City district.
Further details were not immediately available.
Critics say that Prime Minister Nuri al-Maliki, a Shi'ite Muslim, has inflamed sectarian divisions by marginalising Sunnis instead of unifying Iraqis against the insurgents, now called the Islamic State.
Maliki, who has served in a caretaker capacity since the election in April, has said he would seek a third term despite calls from Sunnis, Kurds and even some Shi'ites for him to make way for a less polarising figure.
Some armed Sunni groups are so fiercely opposed to Maliki that they have joined forces with the Islamic State even though they sharply disagree with its radical brand of Islam.
U.S. military and Iraqi security officials estimate the Islamic State has at least 3,000 fighters in Iraq, rising towards 20,000 when new recruits since last month's advance are accounted for.
The Islamic State, an al Qaeda spin-off, has declared a caliphate in parts of Iraq and Syria. It has been systematically stamping out any religious or cultural influences in the major city of Mosul.
The group has also blown up sacred mosque shrines which it deems as non-Islamic, a move which left people weeping in the streets. Women who do not wear full-face veils now risk severe punishment.
Signs are emerging that Iraqis who first welcomed the Islamic State are now growing impatient with its practices.
"I feel frustrated, desperate and sad as a result of the acts and practices done by those who are running the affairs of the city," said shopowner Abu Mohammed.
"I was supporting them at the beginning but when I see what they are doing, I wish we could kick them out of our city." (Additional reporting by Raheem Salman, editing by Mark Heinrich)