More than 900 civilians have fled Mosul and across the border into Syria, the United Nations said Wednesday as U.S.-backed Iraqi forces continued to advance on the ISIS stronghold.

A total of 912 people from the Iraqi city are currently at a U.N.-operated camp in Syria, a spokesman for the Office of UN High Commissioner for Refugees (UNHCR) told NBC News. Local security is keeping the group separate from other refugees at the Al Hol camp, the agency said.

It is the first major group of civilians to have fled Mosul since a military campaign began early Monday to retake the city from ISIS. The terror group took control of Mosul two and half years ago when the Iraqi army collapsed.

Jessem Al-Atlyah, deputy minister of Iraq’s Ministry of Migration and Displacement, said a total of 1,200 people had left the city’s combat areas since since Monday.

“The number of refugees is going to increase in the coming days because Iraqi forces are advancing towards areas that are close to Mosul,” said Al-Atlyah. He added that the ministry had set up refugee camps in various areas with a total of 50,000 tents.

“We are working to increase this number of tents in the coming days,” he said.

So far, the military offensive is making steady progress. Iraqi federal police said forces had defused improvised explosive devices (IEDs) in areas liberated to the south of Mosul. Eighteen villages have been liberated and cleared, federal police said.

Federal police are surrounding the Al-Shura district to the south of the city, according a spokesman for joint operation command, while Kurdish Peshmerga and Iraqi fighters are encircling the Hamdaniyah area to the east.

Officials are optimistic the offensive will be a success in driving ISIS out of its Iraq stronghold, effectively dismantling the self-declared caliphate. ISIS has between 5,000 and 6,000 fighters defending Mosul, the head of Iraq’s special forces Lieutenant General Talib Shaghati said Wednesday.

Related: One Million Could Be Driven from Homes in Battle for Mosul

But humanitarian groups remain concerned for the more than one million people who are still in the city. The Associated Press reported Wednesday that Iraqi and Kurdish authorities had nearly finished work on a refugee camp that can receive some 5,000 families to the east, a sign of the massive number of people expected to flee their homes.

“We are very worried about the safety of innocent civilians in Mosul city,” said Su’ad Jarbawi, Iraq country director for Mercy Corps, a humanitarian aid agency, in a statement. “They will be in the middle of the battleground, and this is only the beginning of a larger humanitarian crisis.”

Mercy Corps is calling on the U.S. to authorize $325 million in emergency supplemental funding to support Iraqi families.