Iraqi government forces captured the major Kurdish-held oil city of Kirkuk on Monday, responding to a Kurdish referendum on independence with a bold lightning strike that transforms the balance of power in the country.

A convoy of armored vehicles from Iraq's elite U.S.-trained Counter-Terrorism Force seized Kirkuk's provincial government headquarters less than a day after the operation began, a Reuters reporter in Kirkuk said.

Neither side gave a casualty toll for the operation. But an aid organization working in Kirkuk said several Kurdish peshmerga fighters and members of the Iraqi forces had been killed in an overnight clash south of Kirkuk — the only serious fighting reported.