BAGHDAD (Reuters) - Up to 78,000 people could be trapped in Islamic State-held Hawija in northern Iraq, the United Nations said on Tuesday, as security forces push to recapture the town. Iraq started an offensive on Sept. 21 to seize Hawija, which fell to the hands of militants after the Iraqi army collapsed in 2014 in the face of the Islamic State offensive and remains the last militant-held town in the country s north. U.N. humanitarian spokesman Jens Laerke said the number of people who have fled the fighting has increased from 7,000 people during the first week of the operation to some 12,500 people now. But up to 78,000 remain trapped, he said.  Iraqi security officials say the militants prevent some residents from leaving, while others are afraid of escaping toward government forces because of the explosives that might have been laid by Islamic State around the town.   We remain concerned for the lives and well-being of these vulnerable civilians and remind those doing the fighting that civilians must be protected at all times and allowed to safely leave Hawija,  Laerke said. Laerke said more people were expected to flee the fighting in areas around Hawija in the next 24 to 48 hours as security forces push into more densely populated areas. Hawija, north of Baghdad, and a stretch of land along the Syrian border, west of the Iraqi capital, are the last stretches of territory in Iraq still in the hands of Islamic State. 