The U.N humanitarian chief on Tuesday said tens of thousands of people are fleeing Syria as the Islamic State militant group expands its grip on large parts of the country despite a U.S.-led airstrike campaign.

Valerie Amos said tens of thousands more civilians could flee if the Islamic State group's fighters continue to seize control of more territory.

"Over the past two weeks, ISIL forces have advanced in northern Aleppo and over 160,000 people, mostly women and children, fled into Turkey in just a few days," Amos told the Security Council, using one of the acronyms for the group. "Their fear is so great that many people crossed heavily mined fields to seek refuge."

More than a week has passed since the United States and Arab allies began airstrikes against the Islamic State group's camps and other assets inside Syria. It has created an unusual situation where the U.S., which opposes Syrian President Bashar Assad, and Syria's government are on the same side against the extremist group. Syria has said the U.S. informed it of the first airstrikes but has given no further notifications as the campaign continues.

The rise of the Islamic State group has complicated Syria's brutal civil war, now in its fourth year, which already has sent three million registered refugees into neighboring countries. Amos told the Security Council that the actual number of people who have left is "far higher than that."

"Those who can, flee," she said simply.

Her monthly briefing on Syria's humanitarian situation remained bleak, despite the opening of the Qamishli border crossing with Turkey that Amos said will help get aid to another 225,000 people. In July, the Security Council unanimously approved the delivery through four border crossings with Turkey, Jordan and Iraq without the approval of the Syrian government, which Amos has blamed repeatedly for slowing the aid process.

The U.N. has blamed both the Syrian government and the armed groups fighting it for what Amos called "utter disregard for international humanitarian and human rights law," with violence on both sides causing civilian deaths.

Amos said 11 million people inside Syria still need urgent aid. She also welcomed the $1 billion pledged last week by various parties during the U.N. General Assembly but said more is needed. "Without additional funds, the World Food Program will be forced to end its operations completely within two months," she said.

Syria's ambassador to the United Nations issued an angry response to her comments. "In all her briefing, I didn't hear the word `terrorist,"' Bashar Ja'afari told reporters. "For her, they are Syrian opposition groups. ... She's still insisting on misreading the facts and reality taking place in Syria."

He criticized the U.N. humanitarian chief for thanking neighboring countries for their help but not his own government, which he said is the source of 75 percent of humanitarian aid.