Iraqi security forces and allied militias on Tuesday repelled two attacks by Islamic State (IS) militants with six suicide car bombs in the provinces of Salahudin and Anbar, security sources said.

In Iraq's northern central province of Salahudin, three suicide truck bombers detonated in the morning at the first defensive line of the security forces and allied paramilitary militias, known as Hashd Shaabi, or popular mobilization, in Harejiyah area, just east of the battleground town of Baiji, some 200 km north of the Iraqi capital of Baghdad, a security source from Salahudin province anonymously told Xinhua.

The powerful blasts were aimed at paving the way for dozens of IS militants to sweep the troops and their bases in the area, the source said.

The troops repelled the attacks as the battles are still underway in the afternoon, the source said.

"Initial military reports put the toll of the security forces at nine killed and more than 23 wounded," the source said, adding that many of the IS militants were killed and wounded but no immediate reports about exact numbers of their casualties.

Security forces and allied militias, backed by Iraqi and U.S.-led coalition aircraft, have cleared most of the town of Baiji earlier in the month after days of heavy clashes with the extremist militants, but the two sides have involved in fierce tug-of-war battles inside and near the town.

The battles in Baiji came as heavy clashes continue in the nearby Iraq's largest oil refinery as the security forces are fighting to drive out IS militants from Baiji refinery, which the militants are seizing large parts of it.

In the country's western province of Anbar, IS militants blew up three suicide truck bombs before dawn on the security forces and Hashd Shaabi paramilitary militias at the edge of Anbar University while dozens of IS militants attacked the troops at the university compound which located in southwestern the provincial capital city of Ramadi, which itself is about 110 km west of Baghdad, a provincial security source anonymously told Xinhua.

The extremist militants also detonated 12 roadside bombs targeting military vehicles when they tried to move at the southern entrance of the compound, destroying several vehicles, the source said.

The troops repelled the IS attacks, but they involved in tug-of-war battles around the vast university compound, while other security forces are trying to expand their foothold outside the compound which the troops seized it two days ago as part of their effort to free the IS-held Ramadi, the source added.

The battles overnight and in the morning left at least 18 IS militants and three suicide bombers killed and 26 others wounded, the source said without giving further details about the casualties among the troops and Hashd Shaabi.

On July 13, the Iraqi authorities announced the start of a major offensive against IS militants to free key cities and towns in Iraq's largest province of Anbar province from IS militants.

Iraqi security forces and allied Hashd Shaabi paramilitary militias have been fighting for months to retake control of key cities and towns in Iraq's largest province of Anbar since the IS militants seized most of it and tried to advance toward capital Baghdad, but several counter attacks by security forces and Shiite militias have pushed them back.

The security situation in Iraq has drastically deteriorated since June 10 last year, when bloody clashes broke out between Iraqi security forces and the IS.