GENEVA (Reuters) - Up to 100,000 Iraqis may flee to Syria and Turkey to escape the Iraqi government’s assault aimed at ousting Islamic State from the northern city of Mosul, the United Nations refugee agency UNHCR said on Monday.

The U.N. High Commissioner for Refugees issued an appeal for an additional $61 million to provide tents, camps, winter items and stoves for displaced inside Iraq and new refugees needing shelter in the two neighboring countries. That brought UNHCR’s overall appeal for the crisis to $196 million.

All civilians fleeing their homes “must be protected and provided with safe passage”, said the International Committee of the Red Cross, which like the UNHCR is based in Geneva. ICRC teams were near Mosul to help 270,000 people with food and other emergency aid, the agency said in tweets.

“UNHCR is concerned that events in Mosul may cause up to

100,000 Iraqis to flee towards Syria and Turkey,” the U.N. agency said. “Preparedness plans are underway in Syria to receive up to 90,000 Iraqi refugees.”

Iraqi government forces, with air and ground support from the U.S.-led coalition, launched an offensive on Monday to drive Islamic State from Mosul, the militants’ last major stronghold in the country.

Over 3 million people, nearly a tenth of the population, are already internally displaced in Iraq. Aid agencies say up to one million could flee Mosul, a city of 1.5 million where in 2014 Islamic State proclaimed a “caliphate” in Iraq and Syria.

U.N. Humanitarian Coordinator for Iraq Lise Grande said in Erbil last week the U.N. was bracing for the world’s biggest and most complex humanitarian effort in the battle for Mosul.

She told Reuters civilians risked being used as human shields or even gassed, noting that Islamic State has said it could use chemical weapons to defend the city.

UNHCR said that a site in Syria that previously hosted Iraqi refugees is being readied to accommodate another influx. Syria is currently home to some 26,000 Iraqi refugees.

Refugees from Mosul were expected to enter Syria at Tel Sefug village, “a crossing point mainly used by smugglers”, along the border with Hasaka province, the UNHCR said. The Damascus government has an “open-door policy” towards refugees.

There were “no clear safe routes out of Mosul”, leaving civilians in danger of getting caught in the cross fire or being victims of booby traps or landmines, the International Rescue Committee said in a statement.

The aid agency voiced concerns that oil burning in trenches around the city - a tactic often used by retreating ISIL troops - will be harmful, especially to children and the elderly.