BAGHDAD, Sept. 21 (Xinhua) -- Clashes continued on Sunday between the Iraqi security forces and insurgent militants, including the Islamic State (IS), an al-Qaida offshoot, in Iraq, security sources said.

　　In Iraq's western province of Anbar, the security forces backed by Shiite militias and allied Sunni tribesmen launched in the morning an offensive aimed at breaking the siege of more than 400 officers and soldiers who have been surrounded by IS, for six days in Saqlawiyah area, just northwest of the militants-seized city of Fallujah, some 50 km west of Baghdad, a provincial police source told Xinhua on condition of anonymity.

　　The troops retook control of part of Saqlawiyah and the nearby al-Sicher areas after they advanced from two directions towards the besieged soldiers and managed to break the siege, but the fierce battles are still underway, the source said without giving further details about the besieged soldiers.

　　Dozens of insurgent militants have been killed and wounded in the clashes, the source said, adding that in a single incident, a warplane bombed a position of IS militants in Sicher area, leaving 13 militants killed, including their local leader.

　　In Salahudin province, heavy air strikes continued against positions of the Islamic State militants in and near the provincial capital city of Tikrit, some 170 km north of Baghdad, a provincial security source told Xinhua on condition of anonymity.

　　The air strikes, which some believed to be carried out by U.S. warplanes, started after midnight and continued to the dawn and pounded the militants' positions in central Tikrit and in the northern edges of the city, the source said.

　　Meanwhile, several U.S. cargo planes and helicopters were seen since the dawn landing in the huge military air base, located some 10 km north of Tikrit, which was known before 2003 U.S.-led invasion as the Air Force College, but the U.S. occupation army used the site as contingency operating base and was known as Camp Speicher, the source said, citing security reports.

　　The U.S. forces apparently are rebuilding a forward operating base to control the combat operations with IS in central and northern Iraq, the source said, adding that such move would be part of cooperation between the Iraqi government and world efforts to fight the extremist IS militants.

　　In June, armed Sunni insurgents, spearheaded by IS, launched a surprise offensive on Iraqi security forces. The group captured a large part of the country's northern and western territories after Iraqi troops abandoned their posts and military equipment. But the insurgent militants failed to seize the huge Camp Speicher air base after some troops fought back the attackers despite the insurgents captured the city of Tikrit to the south.

　　In the country's eastern province of Diyala, heavy clashes broke out between security forces backed by Shiite militias and IS militants in several villages in north of the city of Maqdadiyah, leaving four security members killed and 14 others wounded, including six Shiite militiamen, a provincial security source anonymously told Xinhua.

　　Sadiq al-Husseini, head of the security committee of the provincial council, told Xinhua that up to 25 IS militants have been killed in the clashes and several of their vehicles carrying heavy machine guns were destroyed in the battles.

　　Iraq is witnessing some of its worst violence in recent years. According to the UN Assistance Mission for Iraq, Terrorism and violence have killed 5,576 civilians in Iraq in the first half of this year, with 11,666 more wounded.