Militants of the Islamic State group on Friday seized at least six villages from Syrian rebels near the Turkish border in rapid advances that forced the evacuation of a crucial hospital amid heavy fighting in the area, Syrian opposition activists and an international medical organization said.

The advances in the northern Aleppo province brought the militants to within few kilometers from the border town of Azaz, where rebels hold an enclave that is hosting tens of thousands of internally displaced civilians.

In recent months, Syrian rebel factions in Azaz, and its border crossing of Bab al-Salama, have separately come under fire from the extremist ISIS group, pro-government forces and the predominantly Kurdish Syrian Democratic Forces.

The Britain-based Syrian Observatory for Human Rights said Friday's advance also effectively cut off a key supply route between Azaz and Marea, another opposition stronghold. Both Azaz and the Bab al-Salama crossing have been a lifeline for the opposition since the town fell into rebel hands in 2012, up until recently.

ISIS' news agency, Aamaq, also reported the advance, saying the Islamic State group seized six villages from rebels.

The humanitarian medical organization Doctors Without Borders said its team is currently evacuating patients and staff from the Al Salama hospital, which it runs in Azaz, after the frontline shifted to within 2 miles from the facility.

The group, known by its French acronym MSF, said a small skeleton team will remain behind to stabilize and refer patients to other health facilities in the area.

"MSF has had to evacuate most patients and staff from our hospital as front lines have come too close," said Pablo Marco, MSF operations manager for the Middle East. "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.

"There is nowhere for people to flee to as the fighting gets closer," he said.

A route known as the Azaz corridor links rebel-held eastern Aleppo with Turkey. That has been a lifeline for the rebels, but a government offensive backed by Russian air power and regional militias earlier this year dislodged rebels from parts of Azaz and severed their corridor between the Turkish border and Aleppo.

The predominantly Kurdish Syrian Democratic Forces (SDF), who are fighting for their autonomy in the multilayered conflict, also gained ground against the rebels.

That left the rebels in Aleppo with just one narrow corridor to the outside world, through Idlib province. Those in Azaz are now squeezed between ISIS to the east and the SDF to the west and south, while Turkey tightly restricts the flow of goods and people through the border.

MSF and other aid organizations warned earlier this month that the humanitarian situation for over 100,000 people trapped in the Azaz rebel-held pocket was critical.

On Thursday, Staffan de Mistura, the U.N. envoy for Syria, said he plans for a resumption of peace talks "as soon as feasible" between the government and opposition but that he set no new date and expects that it will "certainly not" come within the next two to three weeks, his office said.

The lack of a firm date for negotiations testifies to continued violence in Syria and difficulties for U.N. efforts to ship humanitarian aid to beleaguered Syrians amid fighting between President Bashar Assad's troops and their allies and rebel fighters. The talks were suspended last month with little to no progress.