The northern province of Aleppo borders Turkey and is criss-crossed with supply routes that are strategic for practically all of Syria's warring sides.
On Saturday, IS fighters seized another border village in their offensive against non-jihadist rebel groups in the province, the Syrian Observatory for Human Rights said.
The IS counterattack has rolled back rebel gains in the border area, which had seen advance them from Azaz, a town eight kilometres (five miles) south of the Turkish border, towards IS bastions further east.
"The IS gains have cut off opposition territory around Azaz from rebels in the town of Dudyan further east," Observatory head Rami Abdel Rahman said.
"Now the rebels in Dudyan are practically surrounded by IS."
The jihadist group also advanced against government forces near Khanasser, a battleground town southeast of Aleppo that has changed hands several times.
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The road through Khanasser is the sole link between government-held areas in and around Aleppo and those in the rest of the country.
On Saturday, IS took several hilltops in the area but had not yet cut the supply route, Abdel Rahman told AFP.
In Aleppo itself, two civilians were killed by rebel fire on a residential neighbourhood in the government-held west of the city, the Observatory said.
Aleppo was once Syria's commercial hub, but since rebels seized eastern districts in 2012 a front line has carved through the heart of the city.
The escalating clashes in Aleppo province have strained a fragile ceasefire in place since February 27, and left more than 200 combatants dead in the past week.
IS and its jihadist rival Al-Qaeda are not party to the truce.
In total, more than 270,000 people have been killed since conflict first erupted in Syria in 2011.