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.
"This would unleash a humanitarian nightmare unlike any seen in the blood-soaked Syrian conflict", he told reporters.
Syrian government forces, backed by Russian Federation and Iran, have been massing troops for weeks around Idlib in preparation for an attack.
On Tuesday, Erdogan said Russian Federation and Iran were also responsible for stopping a humanitarian disaster in Idlib, and said the worldwide community had to "throw its weight behind a political solution".
In Geneva, the United Nations and Russian Federation are discussing a new Syrian constitution, far removed from Idlib, where constitutional rights are powerless to stop a bloodbath.
Turkey has renewed calls for worldwide backing for a ceasefire in Idlib, telling the United Nations security council that an all-out assault on Syria's rebel-held province would trigger a huge wave of refugees and could threaten Europe.
Pope OKs probe into US bishop as he meets with US delegation
No statement was scheduled to be released following the meeting at the Vatican, said a spokeswoman for the Holy See. He said: "In these times , it seems like the 'Great Accuser " has been unchained and has it in for bishops .
Fighters from a coalition of Islamist forces celebrate on after taking the al-Mastumah base in the Syrian city of Idlib.
On Friday at a summit in Tehran, the presidents of Russia, Iran and Turkey could not agree on staving off military action in the province. "It's just not my style", Mattis said when asked if and what kind of retaliation the United States military would carry out should the regime use chemical weapons in its offensive against Idlib.
The threatened assault by President Bashar al-Assad's regime could deprive the few thousand left of their last stronghold in their adopted homeland.
The second factor is the potential use of chemical weapons by Syrian government forces. On the contrary, they would create further suffering, alienate and radicalize more Syrians. "We have seen it in use before, and we strongly advise that it does not happen in this enclosed area, where the population has, I think, nearly doubled by the influx of evacuees and IDPs from other parts of the country".
"We're deeply concerned about this recent escalation of violence, which has resulted in the displacement of over 30,000 in the area. That's something we're monitoring very closely", OCHA spokesman David Swanson told the AFP news agency. The UN has described it as a "dumping ground" for people evacuated and displaced from elsewhere in Syria during the seven-year war.
He said 47 per cent of those displaced have moved to camps, 29 per cent are staying with families, 14 per cent have settled in informal camps and 10 per cent are in rented accommodation.