The National<br/> Syrian rebels defending a key town in Idlib province launched a counter-offensive against advancing regime forces on Thursday as a Turkey-backed rebel group said it was sending reinforcements to the area. Syrian troops captured a string of insurgent-held villages Thursday, moving to just three kilometres away from the key town of Khan Sheikhoun after capturing five villages overnight, the Syrian Observatory for Human Rights said. Khan Shikhoun town lies on a key highway coveted by the regime that runs through Idlib, connecting government-held Damascus with the northern city of Aleppo, which was retaken by loyalists from rebels in December 2016. "The aim of the advance is to surround Khan Sheikhoun and reach the highway," Observatory head Rami Abdul Rahman told Agence France-Presse. The advance towards Khan Sheikhoun also threatens to encircle the last remaining pocket of rebel-held territory in neighbouring Hama province, including the towns of Morek, Kafr Zeita and Latamneh. Government troops reached within have been advancing in a pincer movement along on the southern edges of Idlib province, but rebels launched a counter-attack on the eastern pincer at the village of Sukeik, with dozens were killed in the fighting, according to the Britain-based Observatory, which reports on the conflict using a network of sources. A spokesman for the National Army, a Turkey-backed rebel group, said it was joining forces with the National Liberation Front, another group supported by Ankara, to oppose the regime offensive. “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 (HTS), an extremist faction formerly linked to Al Qaeda. Damascus has struggled to make any gains in the north-west region since launching an offensive in late April. But since the collapse of a brief ceasefire early this month, it has managed to take several significant positions, including the town of Al Habeet on Saturday. There have been reports that Russia has recently stepped up its involvement on the ground and even some reports that Hezbollah – one of the most powerful ground forces in the Syrian war – may be deploying troops after initially saying it had been asked to stay away from the offensive. The Syrian state news agency Sana confirmed a claim by the HTS that it shot down regime fighter plane near Khan Sheikhoun on Wednesday. Sana said the jet was hit with an anti-aircraft missile while on a mission “to destroy headquarters of the Nusra Front”, the former name of HTS. Sana said the pilot’s fate was unknown but the Observatory said a pilot who ejected from the plane had been captured by insurgents. Khan Sheikhoun, which has been in opposition hands since 2014, was hit by a sarin gas attack in 2017 that killed dozens of people. The attack that prompted US President Donald Trump to order a missile strike against the Syrian air base from where the United States said it had been launched. An investigation conducted by the United Nations and the Organisation for the Prohibition of Chemical Weapons said the Syrian government was responsible for releasing sarin on the town on April 4, 2017. Damascus denies using such weapons. Syrian rebels have shot down government planes on several occasions during the war that spiralled out of the uprising against President Bashar Al Assad in 2011. However, the regime’s control of the skies have been instrumental to the government’s survival and the Syrian air force has used indiscriminate bombing of towns and cities to prepare for ground operations. The humanitarian adviser to the UN Special Envoy for Syria said the new surge in violence in the northwest threatened the lives of millions after more than 500 civilians were killed since late April.