WASHINGTON/BEIRUT — The United States on Wednesday called for a new ceasefire in Syria as rival forces clashed in the country's northwest, where the government is waging an offensive on the last big stretch of rebel-held territory.

Syrian President Bashar al-Assad launched his assault late last month, saying rebels had breached an existing ceasefire, triggering a civilian exodus by bombarding Idlib and adjacent areas. The latest clashes mark the biggest escalation since last summer between Assad and his rebel enemies in Idlib province and a belt of territory around it.

Washington late on Tuesday said it had seen signs that Assad may be using chemical weapons, including an alleged chlorine attack, in the latest offensive and warned that it would respond "quickly and appropriately" if this were proven. Assad has denied such allegations throughout the war.

White Helmets members also know as the Syrian Civil Defence putting a body into a bag in a town, said to be Saraqib, Idlib province, Syria on May 22, 2019. Reuters TV

"What we really need in Idlib and throughout the country is a ceasefire," James Jeffrey, U.S. special representative for Syria engagement and special envoy to the global coalition to defeat the ISIS militant group, said at a House of Representatives Foreign Affairs Committee hearing.

"These conflicts, back and forth exchanges .... just put tremendous pressure on civilians, they raise the specter of nation-to-nation clash," he said. "So we're very much engaged in trying to get this stopped and get it back to the ceasefire we had basically since September."

However, fighting raged on Wednesday as rebels sought to roll back army advances in the face of a heavy bombardment, state media, insurgents and a war monitor said. Weeks of air strikes, shelling and clashes have driven some 180,000 people from their homes, raising fears of a new humanitarian disaster.

The Syrian government says it is responding to attacks by al Qaeda-linked militants.

When asked about the use of chemical weapons, Jeffrey said the United States remained vigilant on any potential such attack but said it did not have confirmation.

"We do not at this point have any confirmation that chlorine, which was the substance that was suggested or alleged, has been used. ... We haven't finished our review," he said.

France's foreign ministry said it noted the allegations "with a degree of alarm" and that they needed to be investigated.

The Trump administration has twice bombed Syria over Assad's alleged use of chemical weapons, in April 2017 and April 2018. In September, a senior U.S. official said there was evidence showing chemical weapons were being prepared by Syrian government forces in Idlib.

Jeffrey also said the U.S. was engaging with Russia to help de-escalate the conflict in Idlib. "Despite the recent tragic offensive there, Moscow has reconfirmed, at least on paper, their commitment to the ceasefire deal with the Turks," he said.

Plumes of smoke rising from a town, said to be Saraqib, Idlib province, Syria on May 22, 2019. Reuters TV

"Living under bombardment"

The region, home to an estimated 3 million people, including many who fled other parts of Syria as government forces advanced in recent years, has been partly shielded by a truce agreement since last year, brokered by Russia and Turkey. However, Russia has voiced increasing frustration with what it calls violations of the agreement.

Turkish army posts were established along the front line last year to monitor compliance with the agreement. Turkish Defence Minister Hulusi Akar said on Tuesday they would remain, despite reports in Turkish state media that the Syrian army had carried out attacks near one of them.

Many displaced people, camping on the Turkish border, voiced anger and frustration that Ankara had not done more to help them.

"We can no longer put up with living under bombardment or in the open under the trees," said Abu Abdullah, one of thousands of Syrians in white tents dotted around the rock-strewn olive groves close to the frontier.

Turkey-backed rebels had sent reinforcements on Saturday to the front lines of the insurgent enclave, which is dominated by the jihadist Tahrir al-Sham group, the latest incarnation of the former al Qaeda affiliate Nusra Front.

The war monitor, the Britain-based Syrian Observatory for Human Rights, said at least 100 air strikes hit rebel-held northwest Syria on Wednesday.

Syrian state media reported that the army had stopped a large attack by rebel groups in several places, killing many insurgents.

Rebels said they had recaptured the small town of Kafr Nabuda, which the government said it had taken early this month.

"There are a large number ... killed from Assad's forces ... there are many bodies still on the ground in the town," said Naji Mustafa, a spokesman for the Turkey-backed National Liberation Front.

Mustafa said the army had used chlorine gas when shelling Kubayna, in the mountainous frontline area in the northwest of the rebel enclave, causing choking symptoms among some fighters who were treated at a field hospital.

However, he said that because of the intense bombardment, they had not properly documented the cases.

Italy said on Wednesday that an Italian man held for three years in Syria had been freed.