"We also warn any nation that is determined to impose its will through chemical attacks and inhuman suffering, most especially the outlaw Syrian regime, the United States remains prepared to act if we must", Haley said.
Turkish troops and allied rebels are closing in on the Kurdish enclave of Afrin in northern Syria after weeks of battling the Kurdish YPG militia on the edges of the district, Reuters reported on Saturday.
It said existing basement bomb shelters were so overcrowded that some families were being forced to sleep out in the open, and that at least 70 people had been buried in a park because air strikes made it unsafe to reach the town's cemetery.
"Right now we're getting reports - I don't have evidence that I can show you - but I'm aware of the reports of chlorine gas use", he said before arriving Sunday in Oman.
The civilians used a corridor established by the Syrian army amid military gains that have effectively divided eastern Ghouta into three parts.
State television on Sunday broadcast from the eastern Ghouta town of Mudeira, which state television and a war monitor said the army had captured to link up with units on the other side of the enclave.
Rebels and some eastern Ghouta residents contacted by Reuters have said people there do not want to come back under Assad's rule for fear of persecution, an idea the government says is groundless. It would also be the worst setback for rebels since the opposition was ousted from the eastern half of the city of Aleppo in late 2016 following a similar siege and bombing campaign. "To our knowledge, not one critically sick or wounded person has yet been evacuated".
Last week, as pro-government forces advanced steadily towards urban centres in the heart of the Eastern Ghouta, there were two meetings between Russian officials and Jaysh al-Islam on the outskirts of the enclave observed by the UN.
There was no confirmation from Russian Federation, but the United Nations said there had been talks.
Rebel groups in eastern Ghouta have vowed they will fight on.
The Britain-based monitoring group, which relies on an extensive network of sources on the ground across Syria, said 353,935 people have been killed since 15 March, 2011.
On Monday, Syrian forces pounded two rebel towns in Eastern Ghouta, pressing on with a Russia-backed offensive that has so far allowed government fighters to retake almost 60 percent of the enclave.
The U.N. Security Council has demanded a 30-day halt in fighting across Syria in order to allow those in need of medical care to get help and for humanitarian aid to reach people who need food and supplies.
All three towns are controlled by Islamist rebel group Faylaq al-Rahman, which has repeatedly denied engaging in talks with the regime.
Mattis blamed Russian Federation for Syria's chemical weapons stockpile, which Moscow said it would help eliminate as part of an agreement in 2013, but which the U.S. and worldwide observers said Syria used on civilians a year ago. "The cease-fire has failed".