Battles in and around the Syrian city of Aleppo have killed at least 70 pro-government fighters and more than 80 insurgents after the army launched an offensive there, a monitoring group said yesterday.

The army backed by allied militia had captured areas north of Aleppo on Tuesday in what the Syrian Observatory for Human Rights said was an attempt to encircle the northern city and cut off insurgent supply lines.

Aleppo is at the forefront of clashes between the army and a range of insurgents, including Islamist brigades, al-Qaeda’s Syria wing Nusra Front and Western-backed units battling President Bashar al-Assad. The UN is seeking a ceasefire there, a step towards addressing the crisis in Syria which is about to enter its fifth year.

The advance on Aleppo is the second major offensive by pro-government forces in a week. The army and allied combatants from Lebanon’s Hizbollah group have also launched a large-scale assault in southern Syria against insurgents. Yesterday the main route leading north out of Aleppo to the Turkish border was blocked and under fire by pro-government forces, the Observatory’s founder Rami Abdulrahman said.

“The regime went forward a bit yesterday and the road is still closed,” he said. The army was controlling the route from positions it set up in the villages of Bashkuwi and Sifat on Tuesday on either side of the road, he said.

Insurgents can take another route north but it entails going northwest out of the city and circumnavigating army-held areas before heading north again. “It is the very long way around,” he said. He also said poor weather prevented Syrian air force bombardment yesterday but fighting continued on the ground. Casualties on the government side could be higher because 25 of its combatants were unaccounted for, he said.

Sixty-six Syrian insurgents from various groups were killed in the fighting, as well as at least 20 from the Nusra Front, he said.