Islamic State fighters seized rebel-controlled territory on Friday near Syria's border with Turkey in an advance that one monitoring group called the caliphate's most significant in the region in more than two years.
The Syrian Observatory on Human Rights, a monitoring group based in Britain, said the group pushed it's way further into the province of Aleppo and is threatening to overrun the rebel strongholds of Azaz and Merea, which are located in northern Syria just miles from Turkey's border.
The Islamic State has also gained ground near the city of Aleppo, where forces loyal to Syrian President Bashar al-Assad and anti-government rebels are entrenched in fighting.
Islamic State fighters have advanced to within 3 miles of the town of Azaz, capturing villages and prompting thousands of residents there to flee, according to the Syrian Observatory.
But tens of thousands of people are reportedly trapped between Azaz and Syria's border, and the Islamic State offensive has cut off a supply route to Marea, a city further south held by rebel fighters.
The jihadist group launched the offensive Thursday in response to a potential assault by rebels, some of whom are supported by the United States, Reuters reports.
Doctors Without Borders said it has pulled most of its staff from the Al Salamah hospital near the municipalities because of the offensive. The hospital is the largest of six medical facilities operated by the organization in Syria.
"We are terribly concerned about the fate of our hospital and our patients, and about the estimated 100,000 people trapped between the Turkish border and active front lines," Pablo Marco, the organization's operations manager for the Middle East, said in a statement. "There is nowhere for people to flee to as the fighting gets closer."