The Kurdish regional government in Iraq says it has evidence that Islamic State militants used a chemical weapon against Kurdish peshmerga forces.

The Kurdistan Region Security Council released a statement Saturday alleging that chlorine was used in a January suicide bombing in northern Iraq.

The statement said a lab analysis found chlorine traces in samples from the scene of the attack. The Kurds said the lab was located in a partner nation in the U.S.-led coalition fighting the Islamic State group, but they did not identify the nation or the lab.

The January 23 suicide bombing took place on a road between the Islamic State-held city of Mosul and the Syrian border, according to the statement.

The Islamic State group has been suspected of using chlorine in previous attacks in Iraq and Syria.

Saturday's allegation by the Kurdish government could not be independently verified.

It came as thousands of Iraqi forces and Iranian-backed Shi'ite militia members paused their push to retake Tikrit from the Islamic State group for a second day. Islamic State snipers, roadside bombs and booby traps have slowed down the offensive, which has already succeeded in seizing several areas.

Military commanders were reportedly planning their strategy for taking final control of the city.

Tikrit, just 140 kilometers northwest of Baghdad, has both symbolic and strategic value. It is the hometown of the late dictator Saddam Hussein and is also seen as a key step toward retaking Mosul, Iraq's second-largest city.

At a news conference Saturday in Egypt, U.S. Secretary of State John Kerry said the U.S. knows about and "understands" Iran's involvement in the offensive. He said that while the U.S. does not coordinate with Iran, it knows Iran is also opposed to the Islamic State group.

On Friday, the head of the U.S. Central Intelligence Agency said the militants were well-armed and well-financed but not invincible.

John Brennan told the Council on Foreign Relations in New York that U.S.-led coalition airstrikes had "softened" many of the Islamic State forces and that there were serious signs that the group's "engine is suffering."

"We see there is some dissension in the ranks," he said. Controlling territory and administering it "is not really the strong suit of some of these thugs."