Syrian Observatory says group now in control of 95,000 sq km of land, as Palmyra residents report worsening conditions.
The Islamic State of Iraq and the Levant (ISIL) group has seized more territory from Syrian rebels in Aleppo province near the border with Turkey.
The advance by the armed group on Tuesday threatens to cut off supply lines used by Syrian rebel factions fighting both ISIL and the Syrian government.
The UK-based Syrian Observatory for Human Rights said rebel groups have sent reinforcements to ward off the offensive, which has seen four villages and a town previously held by the al-Qaeda-affiliated Nusra Front fall to ISIL fighters.
A video posted by ISIL on YouTube showed its fighters walking without opposition in the town of Umm al-Qura in Aleppo.
On Tuesday, the pro-ISIL Aamaq News Agency released a video showing fighters with the group as they captured Umm al-Qura. The video showed the fighters marching in fields near the village, as well others shooting from the roof of a building.
“Thank God, with His help the brothers of the Islamic caliphate advanced at several northern countryside villages,” an unidentified ISIL fighter said in the video.
“God has made it easy for us to get hold of their necks and we were able to kill 35 and what is coming is worse for them than this.”
Activists said the reinforcements were being sent from other parts of Aleppo province, as well as from rural Hama and nearby Idlib, where rebels recently captured wide areas from government forces, including the provincial capital.
The fighting came a day after US State Department spokeswoman Marie Harf said authorities heard reports that forces loyal to Syrian President Bashar al-Assad launched air strikes supporting ISIL’s advance.
“Beyond that, we have long seen that the regime avoids ISIL lines, in complete contradiction to the regime’s claims publicly to be fighting ISIL,” Harf said.
“As we have long said, Bashar Assad has lost legitimacy long ago and will never be an effective counterterrorism partner, despite what he might say publicly.”
The Syrian Observatory says that following recent gains in both Homs province in the centre of the country and Aleppo, ISIL now controls half of the country’s land area.