Elite Iraqi military units advanced toward Kirkuk Sunday night, taking key installations in the face of light Kurdish resistance, Iraqi military sources said Monday morning. Kurdistan regional authorities also confirmed the advance, but cited “lots of casualties.”

In the wake of a recent Kurdish vote for independence from Baghdad, the Iraqi parliament authorized the deployment of troops to retake disputed territories. The Kurds had taken advantage of focus on driving back the Islamic State to annex several areas formerly under Iraqi control.

A Kurdish Regional Government official confirmed with the Financial Times that Iraqi forces had taken over headquarters of the Baghdad-controlled North Oil Company, and was moving on the K1 military base. The Iraqi military said it has taken over K1 as well as an airport.

There were casualties on both sides, marking the end to a delicate partnership between Iraqi and Kurdish forces in the fight against ISIS. US officials said they were “engaged with all parties in Iraq to de-escalate tension,” according to the BBC.

The Kurdistan Region Security Council also posted a video on Twitter claiming Kurdish Peshmerga forces destroyed five US-made Humvees used by Iranian-backed Shia forces SW of Kirkuk: