Smoke rises from clashes during a battle with Islamic State militants at the airport of Tal Afar west of Mosul, Iraq November 18, 2016. REUTERS/Thaier Al-Sudani

TIKRIT, Iraq (Reuters) - Islamic State killed seven Sunni tribal fighters who support the Iraqi government and five policemen on Saturday in a town south of Mosul, the insurgents’ last major city stronghold in Iraq, local security sources said.

The tribal fighters and police were gunned down at two fake checkpoints set up by the insurgents in Shirqat, a Sunni town between Mosul and Baghdad, they said.

Islamic State has escalated attacks on forces and officials opposed to its rule as it fights off a military campaign to retake Mosul, the largest city in the “caliphate” it declared in 2014 over parts of Iraq and Syria.

The hardline Sunni group claimed an attack on a Sunni wedding west of Baghdad that killed at least 12 people on Thursday. It staged attacks and bombings over the past weeks in the Sunni towns of Falluja and Rutba, also west of the capital.

Iraqi armed forces began their offensive on Mosul on Oct. 17, with air and ground support from a U.S.-led coalition. Kurdish Peshmerga fighters, Sunni tribes and Iranian-backed Shi’ite paramilitary forces are also taking part.