Islamic State jihadists managed Saturday to storm the defences of one of two remaining rebel-held towns near the Turkish border north of Aleppo, a monitoring group said.
"Islamic State fighters stormed the defences of Marea and heavy battles are ongoing between the movement and rebels in the area," Rami Abdel Rahman, the head of the Britain-based Syrian Observatory for Human Rights, said.
Fighting was taking place in outer parts of the town after Islamic State tried to infiltrate it with its favoured tactic of suicide attacks, he added.
Dozens of combatants have been reported killed on both sides during fighting in and around Marea, 20 kilometers south of the border with Turkey.
The jihadist offensive has placed the town under an effective siege and has threatened the nearby city of Azaz along with the border crossing to Turkey - all of which are controlled by rebels opposed to both Islamic State and the Syrian government.
Nearby areas in Turkey have again come under rocket fire since Friday, apparently from Islamic State-held areas in Syria.
The United Nations Refugee Agency (UNHCR) said Saturday it is deeply concerned about the plight of some 165,000 displaced persons reportedly massing near the Syrian town of Azaz in northern Syria.
"People have started to flee due to heavy fighting in northern Aleppo. Fleeing civilians are being caught in cross-fire and are facing challenges to access medical services, food, water and safety," it said in a statement.
The offensive comes even as Islamic State is under pressure in north-eastern Syria, after Kurdish and allied Arab forces backed by the United States launched an offenive this week against areas held by the group north of their de facto capital al-Raqqa.