Islamic State militants have killed 25 members of a Sunni Muslim tribe during their assault on a provincial capital west of Baghdad, local officials said on Saturday, in apparent revenge for tribal opposition to the radical Islamists.

They said the bodies of the men from the Albu Fahd tribe were discovered after the army launched a counter-offensive on Saturday against the Islamic State in a village on the eastern edge of Ramadi, capital of Anbar province.

"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.

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.

Islamic State, which has seized control of large parts of Syria and Iraq, continues to gain territory in Anbar despite three months of U.S.-led air strikes launched against the group.

On Friday it launched coordinated attacks in central and outlying areas of Ramadi in an attempt to take full control over a city which is already mostly in its hands.

Gunmen opened fire from rooftops at a complex in the heart of Ramadi which houses the governorate and police headquarters, but local officials said security forces managed to hold off Islamic State fighters trying to advance toward the buildings.

The road from Ramadi to the military air base of Habbaniya, about 25 km (15 miles) to the east, remained under Islamic State control, Hathal Fahdawi said, preventing the army from reinforcing security forces in the city.

He said tribal fighters backed by army tanks were trying to secure the road to allow forces through from Habbaniya.

Islamic State's lightning offensive through northern Iraq in June plunged the country into its gravest security crisis since the U.S.-led overthrow of Saddam Hussein in 2003, and raised concerns that its radical ideology will spread across the Middle East.