Deadly clashes erupted overnight as jihadist fighters attacked two besieged regime-controlled villages in northwestern Syria, a monitor said on Sunday.
The strike came a day after militants from Al Qaeda's former Syrian affiliate attacked the villages of Fuaa and Kafraya, which are held by forces loyal to Syrian President Bashar Al Assad.
Syrian state news agency SANA also reported Saturday's attack, and said local fighters were able to push back the jihadists.
The UN, he said, has the capacity to send in at least three convoys a week, so it has made only 11 percent of possible deliveries this year - "the lowest ever" since the start of the conflict.More news: Amazon halts new construction; Seattle backs down on business tax
Idlib is home to almost 1 million children, many of them displaced from other parts of the country.
Fuaa and Kafraya are the only two places in Syria now designated as besieged by the United Nations.
"The Secretary-General expresses deep concern about the airstrikes that reportedly targeted the village of Zardana in northern rural Idlib on the night of 7 to 8 June, killing dozens including children".
Moumtzis said that a major battle for Idlib could be "much more complicated and brutal" than fighting for eastern Aleppo and eastern Ghouta - two opposition-held areas that surrendered with evacuations in 2016 and 2018, respectively.
They came under siege in 2015, when rebels ousted regime forces from a vast majority of Idlib province.More news: Trump's vow to end military drills with South Korea stuns a region
Last week, clashes in Idlib between HTS and an IS sleeper cell made up of Iraqi jihadists left more than two dozen fighters dead, mostly from IS.
In retaliation, the IS cell executed five HTS fighters it had abducted in Idlib, the Observatory said.
Troops have employed the tactic alongside heavy bombing to cut off food and medicine to rebel-held areas, then coerce people to agree to leave in population transfers. HTS also killed six Daesh members it had captured in the clashes.
He said Idlib is part of the "de-escalation" agreement for Syria reached between Turkey, Russia and Iran, and warned of the dire consequences if the province, which has some 2.5 million inhabitants, sinks into full-blown conflict.
Idlib, which remains under opposition control, was designated as a "de-escalation zone" in which acts of aggression are expressly forbidden in May of a year ago.More news: Toyota Motor to invest US$1 billion in ride-hailing firm Grab