"The renewed military offensive in Idlib by the Syrian regime and its backers, causing enormous human suffering, is unacceptable," said the EU council, which represents the EU's 27 member states.
"The EU urges all parties to the conflict to fully respect their obligations under international humanitarian law and international human rights law and to allow unimpeded and direct humanitarian access to all those in need," the statement added.
The warning came during an EU summit devoted to drawing up an EU budget.
As he arrived for the summit, French President Emmanuel Macron said events in northwest Syria, where President Bashar al-Assad's Russian-backed forces are seeking to triumph in the country's nine-year civil war, could not be ignored.
"For weeks one of the worst humanitarian dramas has been unfolding," Macron said.
"I wish to condemn in the strongest terms the military attacks carried out for several weeks by Bashar al-Assad's regime against the civilian population of Idlib."
Macron called on the UN Security Council - which France is a permanent member of - to "take responsibility" after Russia blocked it adopting a statement calling for a ceasefire.
Syrian aid workers are urging a ceasefire and international help for nearly a million people fleeing the regime's onslaught in Idlib - the biggest wave of displaced civilians in the conflict.
The UN said 900,000 people had been displaced in "horrendous conditions" since December 1, more than 500,000 of them children.
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