Jihadist forums and social media sites were filled with claims that extremist fighters had executed 1,700 people they described as apostates, underscoring the ever-more sectarian nature of the conflict in Iraq.

The claims, which couldn’t be independently verified, were accompanied by photos of the purported mass shootings carried out by militants from the Islamic State in Iraq and the Levant. The photos recalled some of the footage from the civil war in Syria, where about 160,000 people have been killed, many of them civilians.

Iraq’s government is seeking to regain territory held by the Sunni militant group, an offshoot of al-Qaida. Its advance after capturing the city of Mosul last week put in doubt Prime Minister Nouri al-Maliki’s rule over a unified Iraq -- OPEC’s second-largest oil producer.

The U.S. invasion of Iraq in 2003, and the subsequent rise to power by the Shiite-Muslim majority, alienated their Sunni counterparts, who dominated the country during Saddam Hussein’s era. Shiite Muslims have political and religious ties to Iran, while their Sunni counterparts have felt marginalized from the country’s political process under al-Maliki.

The Sunni Muslims are a majority in the Anbar Province to the east and in areas to the north of Baghdad. The Shiite are the majority in the south, where 60 percent of the country’s oil wealth resides.