A rebel group backed by Turkey will send reinforcements to the front lines of the last major rebel enclave in northern Syria to oppose a government offensive, it said on Thursday. Over the past week, the Syrian army has advanced toward the town of Khan Sheikhoun in Idlib in a pincer movement that could encircle the southern part of the rebel enclave. The British-based Syrian Observatory for Human Rights said on Thursday that the rebels had launched a counterattack on the eastern pincer at the village of Sukeik, and that dozens had been killed in the fighting. The National Army and National Liberation Front, rebel groups that are both supported by Turkey, have joined forces to oppose the offensive. The National Army will send more of its fighters to the front lines to oppose the government units, its spokesman said on Thursday. "It was decided to start sending troops from the National Army starting tomorrow," spokesman Major Youssef Hamoud said. While the National Liberation Front normally operates in Idlib, the National Army's strongholds are located close to the Turkish frontier in an area north of Aleppo. The most powerful group in Idlib is widely seen to be Hayat Tahrir Al-Sham, a militant faction. Colonel Mustafa Bakour, a commander in the Jaish Al-Izza rebel faction, which is also active in the conflict area, said that rebel forces had advanced against government forces in the east of the province. On Wednesday, rebel fighters shot down a Syrian military Sukhoi 22 jet near Khan Sheikhoun, a town that was hit by a sarin poison gas attack in 2017. Eight years of war in Syria have killed hundreds of thousands of people and driven half the pre-war population of 22 million from their homes, including more than 6 million as refugees to neighboring countries. Since Russia joined the war on his side in 2015, President Bashar Assad has managed to retake most of the country, crushing rebel enclaves in all the major cities and driving them from the south. The United Nations and aid agencies have warned of a new humanitarian catastrophe in northwest Syria, as hundreds of thousands of people have fled their homes since the government's offensive began in late April. On Wednesday a US agency which supports health facilities in opposition-held areas of Syria said government air strikes had targeted an ambulance center, killing a paramedic, an ambulance driver and a rescue worker who was trying to free them from the rubble. Mark Cutts, a U.N. official working on Syria, condemned the strike in a statement, saying it "highlights again the horror of what's going on in Idlib and northern Hama". — Reuters