"With all our generosity, we have gone to extraordinary lengths to alleviate suffering of the Syrian people, hosting some 3.5 million refugees, more than any other country in the world".
The comments came four days after the Turkish president met his Russian and Iranian counterparts for a summit in Tehran, where Erdogan sought to avert a bloody assault in Idlib.
Pointing to a United States missile strike on a Syrian air base in April 2017 after Assad's jets launched an alleged chemical attack in Idlib, Mattis noted that the Syrian leader had suffered massive losses to his air force.
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.
Underscoring the necessity of preventing civilian casualties, he further called on Russia, Iran, the US, France and the United Kingdom as well as all other partners and actors in Syria to work together and find a peaceful solution. "We can not leave the Syrian people to the mercy of Bashar Assad", Erdogan wrote in the WSJ.
"Innocent people must not be sacrificed in the name of fighting terrorism", he wrote.
The moderate opposition, along with the Hayat Tahrir al-Sham group, now share control of Idlib's de-escalation zone, in which acts of aggression are expressly prohibited.
Last month, 14 moderate opposition factions unified under a single umbrella group dubbed the National Liberation Front (NLF).
Erdogan acknowledged that groups such as HTS "remain active in this area" but insisted that these fighters "account for a fraction of Idlib's population".
The forces of President Bashar Assad used the banned chemical weapons in attacks on Douma, near Damascus, on January 22 and February 1, as well as in the northern Idlib province on February 4, the report quoted by Reuters said.
Assad's forces began amassing in the areas of Idlib already under its control and on its southern and eastern flanks.
As Russia continues to bombard the northwestern Syrian province of Idlib from the skies, in support of the country's ruling government's campaign to retake territory, Washington is warning of retaliatory strikes.
Haley said the failed summit was another indication that Russia, Iran and the Assad regime are not interested in a political solution, telling the council that those parties have "had every opportunity to demonstrate their credibility as constructive actors in Syria".
A second strike, in April this year, saw British and French militaries assist the US-led mission.
Russia, Turkey and Iran have been working together as "guarantors" for a series of talks around ending Syria's war.