At least 35 persons were killed and 28 injured in a series of attacks carried out by the Islamic State militant group in the Iraqi city of Tikrit, a security source said on Wednesday.Most of the victims in the Tuesday night attacks in Salahudin province were civilians, EFE news quoted the source as saying.The IS militants, disguised in military uniform, attacked security checkpoints and stormed several houses in Zuhour neighbourhood, Xinhua news agency reported.The attackers broke into the houses and opened fire at families, a source said, adding that at least one of the attacked houses belonged to the family of a police officer.Iraqi security forces surrounded the area to prevent them from moving to adjacent neighbourhoods, while fierce clashes with the attackers broke out in Zuhour, the source said.At least five IS militants were killed, the source said.Salahudin Governor imposed a curfew in the city on Wednesday morning.The IS targets security forces, civilians and crowded markets such as cafes and mosques across Iraq.The attacks came as the Iraqi security forces, backed by anti-IS international coalition, are carrying out a major offensive to drive out the IS from their major stronghold in the western side of Mosul.