The United States called on Wednesday for a new ceasefire in Syria as rival forces clashed in the countrys northwest, where the regime is waging an offensive on the last big stretch of opposition-held territory. "What we really need in Idlib and throughout the country is a ceasefire," said James Jeffrey, the US special representative for Syria. "We are very much engaged in trying to get this stopped," he added in a House of Representatives committee hearing. Syrian regime leader Bashar Assad launched his assault late last month saying opposition factions had breached an existing ceasefire, triggering a civilian exodus by bombarding Idlib and adjacent areas. Washington has said it sees signs that Assad has used poison gas in the latest offensive. He has denied such allegations throughout the war. Fighting raged on Wednesday as opposition factions sought to roll back regime advances in the face of a heavy bombardment, state media, insurgents and a war monitor said. Weeks of airstrikes, shelling and fighting have driven at least 180,000 people from their homes, raising fears of a new humanitarian disaster. 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 opposition factions had sent reinforcements on Saturday to the frontlines of the Idlib, which is dominated by the extremist 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 northwest Syria on Wednesday. Syrian state media reported that the army had stopped a large attack by opposition groups in several places, killing many fighters. The US State Department on Tuesday said it saw signs that the Syrian regime may be using chemical weapons, including an alleged chlorine attack on Sunday. The French foreign ministry said the new allegations must be looked into. Naji Mustafa, a spokesman for the Turkey-backed National Liberation Front, said the regime had used chlorine gas when shelling Kubayna, in the mountainous frontline area in the northwest of the opposition 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.