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 Sunni militants. Security sources and residents said the attack on Samarra was launched at 5.30 am (0230 GMT) 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. Medical sources said Samarra hospital received the bodies of 14 Shi'ite militia fighters and policemen. Residents reported seeing black smoke over parts of the city and hearing powerful explosions. After heavy clashes in the morning, the fighting appeared to have subsided by the afternoon. Thousands of troops and fighters from Shi'ite militias known as Hashid Shaabi (Popular Mobilisation) 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. The army shelled northern and western districts of Tikrit on Saturday, but did not send troops into the city, security sources said. Army helicopters had also fired rockets at Islamic State militants around Sur Shnas, they said. 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. Further east in Diyala province, 11 people were killed in a twin car bombing in the town of Balad Roz on Saturday. One of those killed was a judge, security and medical sources said. The army and Shi'ite militias have driven Islamic State out of nearly all of Diyala province, which lies to the north-east of Baghdad, on the border with Iran. But Saturday's blasts showed that militants could still launch attacks there, just as they regularly do in the Iraqi capital itself.