A deadline for the Syrian regime to hand over a list of its banned chemical weapons was also fast approaching.
Iranian President Hassan Rowhani, meanwhile, offered to broker talks between the opposition and the Islamic republic's government allies in Damascus.
After the latest round of a war-within-a-war between foes of the Syrian regime, the opposition National Coalition accused Al-Qaeda front group the Islamic State of Iraq and the Levant (ISIS) of violating the principles of the revolution.
The Northern Storm brigade, which is loyal to the FSA and was based in Azaz, agreed to the truce with ISIS under which both sides pledged to observe a ceasefire, the Syrian Observatory for Human Rights said.
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The deal was brokered by Liwa al-Tawhid, a powerful rebel brigade loyal to the FSA, which sent fighters to the town yesterday who have deployed between the two sides, the NGO said.
Azaz has symbolic as well as strategic value as it was one of the first towns to be captured from government troops, in July 2012, by FSA fighters, who set up their own administration.
Tensions have spiralled between some mainstream rebel groups and ISIS in recent months, especially in northern Syria, where the opposition controls vast swathes of territory.
In the wider conflict, Syria's deputy premier said Damascus believes the 30-month-old war in his country has reached a stalemate and would call for a ceasefire if long-delayed peace talks in Geneva were to take place.
When asked what his government would propose at the stalled Geneva II summit, he replied: "An end to external intervention, a ceasefire and the launching of a peaceful political process.