Idlib is home to around 3 million people, around half refugees who fled other parts of Syria following previous regime assaults.
"Fighting terrorism does not absolve warring parties of their core obligations under worldwide law", said Guterres, who directly appealed to Iran, Russia and Turkey to spare no effort to protect civilians, "preserve basic services such as hospitals, ensure full respect for global humanitarian law".
A full-scale military offensive on Idlib risks creating the worst humanitarian tragedy of the 21st century, United Nations aid agencies have said, amid concerns that a severe funding shortfall threatens the most vulnerable victims of the conflict.
Other aid organisations also operate in the area, but their support is mostly limited to providing supplies to civilians already living in the existing border camps.
The US has conducted two recent high-profile airstrikes in Syria in response to alleged chemical weapons attacks, once in April 2017 and again in April of this year.
The US ambassador, Nikki Haley, said Syrian forces, backed by Russian Federation and Iran, had already launched 100 airstrikes on Idlib this month and that their sole aim was "a bloody military conquest of Idlib". "We can not leave the Syrian people to the mercy of Bashar Assad", Erdogan wrote in the WSJ.More news: The OnePlus 6T won't have a headphone jack
The Turkish leader also criticized Assad's bid to legitimize the fight in Idlib as a counter-terrorism operation.
He told council members there is "an urgent need" to separate opposition fighters seeking a political settlement from the "terrorists" in Idlib, and appealed to those with influence to exert pressure to achieve this.
Additionally, pockets of the province still remain under ISIS control.
Turkey, which now hosts some 3.5 million refugees, has also said that it could not accommodate any more migrants if an attack on Idlib caused a new surge of refugees toward its border. The terrorist organization is controlling almost 60% of Idlib, the place where anti-Assad organisations deployed.
President Bashar al-Assad has now set his sights on Idlib, and his forces have stepped up bombardment of the densely populated province since the beginning of the month.
The United Nations Security Council has been briefed several times, since the tripartite talks in Tehran failed to reach a ceasefire agreement. "The world will hold them responsible", she added. "And no number of Security Council meetings will ever change that".
"The formation process of a constitutional committee will be discussed, and we can say that the preparations have come to the last stage", he added.