Iraq's special operations forces fight ISIS. Reuters/Stringer

(Reuters) -- Islamic State militants killed 25 members of a Sunni Muslim tribe during their assault on a provincial capital west of Baghdad, local officials said Saturday. The attack apparently constituted revenge for tribal opposition to the radical Islamists.

The officials said bodies of men from the Albu Fahd tribe were discovered after the army launched a counteroffensive Saturday against the militant group formerly known as ISIS in a village on the eastern edge of Ramadi, capital of Anbar province.

The killings echoed the execution of hundreds of members of the Albu Nimr tribe last month by Islamic State fighters trying to break local resistance to their advances in Anbar, a Sunni Muslim province they have largely controlled for nearly a year.

“While they were combing the territories they are liberating, security forces found 25 corpses in the Shujariya area,” Hathal Al-Fahdawi, a member of the Anbar Provincial Council, told Reuters.

Albu Fahd tribal leader Sheikh Rafie al-Fahdawi said at least 25 bodies had been found and said he expected the total to be significantly higher. He said the bodies were found scattered around with no signs of weapons next to them, suggesting they were not killed during fighting.

Already in control of large parts of Iraq and Syria, the Islamic State group continues to gain territory in Anbar despite three months of U.S.-led airstrikes launched against the group. It launched coordinated attacks in central and outlying areas of Ramadi Friday in an attempt to take full control over a city that is already mostly in its hands.

(Reporting by Raheem Salman; Writing by Dominic Evans; Editing by Michael Georgy and Alison Williams)