BEIRUT — Militants of the Islamic State group on Friday seized a string of villages from Syrian rebels near the Turkish border in rapid advances that forced the evacuation of a hospital and trapped tens of thousands of people amid heavy fighting, Syrian opposition activists and an international medical organization said.

The advances in the northern Aleppo province brought the militants to within 2 miles of the rebel-held town of Azaz and cut off supplies to Marea farther south, another rebel stronghold north of Aleppo city.

They also demonstrated the Islamic State group's ability to stage major offensives and capture new areas, despite a string of recent losses in Syria and Iraq.

The Britain-based Syrian Observatory for Human Rights, which monitors the conflict through a network of activists on the ground, said Friday's advance was the biggest by Islamic State in Aleppo province in two years.

Human Rights Watch said around 165,000 civilians are trapped near the Turkish border as a result of the fighting.

Turkey has closed its borders with Syria for the past 15 months, and HRW said Turkish border guards enforcing the closure have at times shot at and assaulted Syrian asylum seekers as they try to reach safety in Turkey — charges the Turkish government denies.

“While the world speaks about fighting ISIS, their silence is deafening when it comes to the basic rights of those fleeing ISIS,” Gerry Simpson, senior researcher with the group's refugee program, wrote in a dispatch, using an acronym for Islamic State.

The Islamic State offensive began Thursday night. By Friday, the group had captured six villages east of Azaz including Kaljibrin, cutting off rebels in Marea from the Azaz pocket.

The rebels in the area — which include mainstream opposition fighters known as the Free Syrian Army along with some ultraconservative Islamic insurgent factions — have been squeezed between Islamic State to the east and predominantly Kurdish forces to the west and south, while Turkey restricts the flow of goods and people through the border.

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

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

Al Salama hospital is the largest of six medical facilities run by MSF in Syria.

Azaz, which hosts tens of thousands of internally displaced people, lies north of Aleppo city, which has been divided between a rebel-held east and government-held west.

A route known as the Azaz corridor links rebel-held eastern Aleppo with Turkey. That has been a lifeline for the rebels since 2012, 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.

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

Elsewhere in Aleppo, more than 30 people including 10 children were killed in airstrikes on the rebel-held towns of Anadan and Hraytan just north of Aleppo city, opposition activists said.

Also Friday, the U.N. refugee agency reported a “spike” in the number of Iraqis trying to flee into Syria to escape the Iraqi city of Mosul, controlled by the Islamic State group.

UNHCR spokeswoman Melissa Fleming said, “Just picture this: We have refugees fleeing to Syria” — now in its sixth year of civil war.

The agency said 4,300 arrived at al-Hol camp in Syria's northeastern Hassakeh governorate in May.