At least 21 people, including two children, were killed and 80 others wounded Thursday by suspected rebel shelling targeting a number of districts in the country's northwestern province of Aleppo, the state news agency SANA reported. The bombardment struck the districts of Sulaimaniyeh, Midan and Maysalun in Aleppo and caused material damage, said the agency, adding that some of those wounded by the attack are in critical condition. Meanwhile, the pro-government al-Khabar news website said the rebels' shelling on the same targeted areas in Aleppo is still ongoing, as other local media reports said the Syrian troops foiled the attack of armed militant groups against the Hanano military outpost in Aleppo on Thursday. The incident is the latest in a string of similar shelling attacks by suspected militant groups, which have used such tactics to shake the government's grip on the area and to retaliate against the Syrian army's advancement on many fronts in central and southern Syria. A day earlier, the Syrian Observatory for Human Rights, a London-based watchdog group that relies on a network of activists on ground in the country, said that Syrian government aircraft dropped explosive-laden barrels on rebel-held areas in Aleppo, but did not report any casualties. Aleppo, Syria's largest city and once an economic hub, has emerged as a main battlefield in the country's three-year crisis since the rebels vowed to "liberate" Aleppo from the government forces in June 2012. Meanwhile, the state news agency said the Syrian troops killed many suspected rebels in the towns of Sheikh Miskeen, Tsil, Naiymeh and Musaifra in the southern province of Daraa. Government troops also ambushed another insurgent group in the northwestern province of Idlib, according to SANA. The three-year crisis started in mid-March 2011 when anti- government protesters took to the streets calling for reforms, but rapidly evolved into a civil war which witnessed the formation of anti-government militias joined by radical jihadist movements. Reports estimate that more than 150,000 people have been killed so far.