Islamic State fighters advanced on a key supply route for Syrian rebels near Syria's border with Turkey, causing thousands of Syrians to flee toward the closed border to join tens of thousands already living in informal camps there.
Human Rights Watch (HRW) says 45,000 Syrians fled the ISIS assault between the 24 and 27 May, joining the 120,000 Syrians already "scattered in overcrowded informal settlements and fields just south of Turkey's Öncup?nar/Bab al-Salameh border crossing and in and around the nearby Syrian town of Azaz."
"Aid agencies say there is no question all 165,000 would seek asylum in Turkey if the border were open to them," the HRW statement said.
The UK-based Syrian Observatory for Human Rights said Friday's advance was the biggest by IS in Aleppo province for two years. It brought the jihadists to within 3 miles of Azaz, a town near the border with Turkey through which insurgents have been supplied.
Islamic State said in an online statement it had captured several villages near Azaz, which cut rebel supply lines from there to the town of Marea farther southeast, isolating the latter from other rebel-held areas, the Observatory said.
The IS advance squeezes the corridor of rebel-held territory that leads from the Turkish border toward Aleppo city, which is divided between insurgent and government control.
International medical charity Medecins Sans Frontieres said it evacuated patients and staff from a hospital in the area as the fighting got closer.
In April, IS fighters seized another strategic town near the Turkish border from rebel factions fighting under the banner of the Free Syrian Army.
Islamic State (IS) has been fighting against rebels in the area for several months. The rebels last month staged a major push against IS, but the group counter-attacked and beat them back.
The United States has identified the area north of Syria's former commercial hub Aleppo as a priority in the fight against IS.
Aleppo's northern countryside is the theater of several separate battles between multiple warring sides in the five-year Syrian conflict, which has drawn in military involvement of regional and world powers that back different groups.
Rebels supplied through Turkey have been fighting Islamic State and separately battling Kurdish forces in other areas.
Ankara is concerned by Kurdish advances along its border, where the Kurdish YPG militia already controls an uninterrupted 250-mile stretch.
The United States supports the YPG and allied fighters in its battle against Islamic State farther east, including in Hasaka and Raqqa provinces.
Earlier this week, a US-backed alliance of Syrian militias launched a new offensive against IS fighters near the group's de facto capital of Raqqa city on Tuesday.
The Syria Democratic Forces (SDF) alliance, which includes the YPG, is the main Syrian partner for the US-led alliance battling IS. Aided by US-led airstrikes, the YPG has driven IS from wide areas of northern Syria over the last year or more, though its advances have recently slowed. There has been no indication of when a full assault on Raqqa city might take place.
Separately, al-Qaeda's Syrian affiliate Nusra Front and other insurgents late on Thursday seized control of a town south of Damascus from government forces.
Nusra Front said in a statement it had captured Deir Khabiyeh, which is near an area where government forces and allies have sought to tighten control of a road leading south.
Last week, government forces and Lebanese Hezbollah captured territory in Damascus's eastern suburbs from insurgents.
Nusra Front and IS are rivals in the Syrian conflict and have been fighting each other, including near Damascus, in separate battles from those between insurgents and government forces.
Reuters Contributed to this article.