WASHINGTON, March 16-- The United States on Monday expressed concerns about alleged use of chlorine gas by the extremist group the Islamic States (IS) against Kurdish forces.

"We are certainly concerned about it. We have not been able to independently confirm it," said Pentagon spokesman Army Colonel Steven Warren.

Iraqi Kurdish forces said on Saturday that they had proof that the IS, which occupied a vast area of Northern and Western Iraq, used chlorine gas against Kurdish forces in a car bombing attack in January.

"We have seen what the Kurds have to say. We have no reason not to believe them," Warren said.

Some 30,000 Iraqi troops and thousands of allied Shiite and Sunni militias have been involved in a week-long offensive to recapture Tikrit and other key towns and villages in northern part of the Salahudin province from IS militants.

Large parts of the province have been under IS control since June 2014 after bloody clashes broke out between Iraqi security forces and the IS group.

The IS has taken control of the country's northern city of Mosul and later seized swathes of territories in Nineveh and other predominantly Sunni provinces.