The clashes on several fronts have put a strain on a fragile ceasefire in place since February 27, and left more than 200 fighters on all sides of the civil war dead in recent days.
The delegation representing President Bashar al-Assad's regime arrived today in Geneva where UN-brokered indirect talks between representatives of the government and opposition were due to be held.
However, the fighting around Syria's second city Aleppo cast a shadow over international efforts to end the five-year war, which has left more than 270,000 people dead and forced millions to flee their homes.
They also battled jihadists from the al-Qaeda-affiliated Al-Nusra Front and allied rebels in the flashpoint area of Handarat north of Aleppo city, it said.
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Meanwhile IS fought rebels near the Turkish border, the Britain-based monitor added.
"What is happening in Aleppo is a major violation of the ceasefire," rebel commander Major Eyad Shamsi told AFP in Geneva, blaming the regime.
"A big battle is being fought in Aleppo, and it will lead to a major disaster should the regime succeed" in cutting off the route linking rebel-held parts of Aleppo to the northern countryside, he said.
On one Aleppo front alone -- pitting rebels against IS -- fighting has forced about 30,000 civilians to flee, according to Human Rights Watch.
The watchdog accused Turkish border guards of shooting at some of those displaced as they approached the frontier.
"As civilians flee ISIS fighters, Turkey is responding with live ammunition instead of compassion," said HRW researcher Gerry Simpson, using another acronym for the jihadists.
"The whole world is talking about fighting ISIS, and yet those most at risk of becoming victims of its horrific abuses are trapped on the wrong side of a concrete wall.