More than 100,000 Syrians were trapped near the Turkish border as fighters from the Islamic State of Iraq and the Levant (ISIL) group advanced on two strategically-vital towns.

Doctors Without Borders (MSF) evacuated most of its staff and patients from al-Salamah hospital - the organisation's largest facility in the country - near the town of Azaz in Aleppo province on Friday as ISIL neared.

"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 frontlines," Pablo Marco, MSF operations manager for the Middle East, said in a statement.

"For some months, the frontline has been around seven kilometres away from the hospital. Now it is only three kilometres from al-Salamah town. There is nowhere for people to flee to as the fighting gets closer."

ISIL, also known as ISIS, has cut off a key road between rebel-held Azaz, close to the Turkish border, and nearby Marea, journalist Maamoun Khateeb told the AFP news agency from Azaz.

More than 100,000 Syrians flee as ISIL advances

"This is a disaster," Khateeb said, adding that some 15,000 people were now besieged in Marea.

Turkey has closed its border to all but seriously wounded Syrians.

Marea and Azaz fell to opposition forces in 2012 and have been vital stops along a rebel supply route from Turkey.

ISIL has tried to advance on the towns for months. In a statement on Friday, it said it had launched a "surprise attack" and seized a series of villages near Azaz.

Patchwork of territories

Also on Friday, a government bombardment on rebel-controlled areas in Aleppo province left at least 15 people dead, rescue workers told AFP.

At least two people were killed in barrel bomb attacks on an opposition-controlled eastern district of Aleppo city, a civil defence group known as the White Helmets said.

Air strikes also killed nine people in the town of Hreitan and four in Kfra Hamra.

READ MORE: Aleppo patients 'have nowhere to go'

Since fighting intensified there in 2012, Aleppo province has been transformed into a patchwork of territories held by the government, rebels, Kurds and other fighters.

Gerry Simpson, a researcher at Human Rights Watch, said Turkey should open the border and allow safe passage to those fleeing the ISIL offensive.

"The fact Turkey is generously hosting more than 2.5 million Syrians does not give it a right to shut its border to other endangered Syrians," he wrote in a HRW statement on Friday.

Staffan de Mistura, UN special envoy for Syria, has estimated that more than 400,000 have been killed in the five-year war. The UN stopped keeping an official count in 2014.

More than 4.8 million Syrians have become refugees in neighbouring countries - Lebanon, Jordan, Turkey and Iraq - as well as Europe, according to the UNHCR. At least 7.6 million Syrians have been forced from their homes within the country's borders.