BAGHDAD (Reuters) - Islamic State suicide bombers and fighters struck targets on Saturday in the northern Iraqi city of Samarra, where security forces and their Shi'ite militia allies have been gathering for an offensive against the radical militants.

Security sources and residents said the attack on Samarra was launched at 5.30 am (9.30 p.m. ET Friday) when two Islamic State suicide bombers blew up their cars in the northern area of Sur Shnas.

At the same time a man drove a Humvee rigged with explosives into the south of the city and detonated it, while Islamic State fighters attacked security forces to the west with sniper fire, mortars and rocket propelled grenades.

Thousands of troops and fighters from Shi'ite militias known as Hashid Shaabi (Popular Mobilization) have gathered around Samarra for a campaign to drive Islamic State out of nearby strongholds on the Tigris River, including the city of Tikrit 50 km (30 miles) to the north.

Medical sources said Samarra hospital had received the bodies of three Hashid Shaabi fighters, and was treating six wounded people.

Residents reported seeing black smoke over parts of the city and hearing powerful explosions, as clashes continued.

In the town of Ishaaqi, about 20 km (10 miles) southeast of Samarra, snipers shot dead two Hashid Shaabi men as they tried to set up a sand barrier on the main highway linking Samarra to the capital Baghdad.

(Reporting by Ahmed Rasheed; Writing by Dominic Evans; Editing by Pravin Char)