Beirut, Reuters—The Syrian army backed by allied militia have captured several villages near Aleppo in battles aimed at encircling the northern city and cutting off insurgent supply lines, a monitoring group said on Tuesday.

The advance on Aleppo is the second major offensive by pro-government forces in a week. The army and allied combatants from Lebanon’s Hezbollah group have also launched a large-scale assault in southern Syria against insurgents.

The UK-based Syrian Observatory for Human Rights said heavy battles north of Aleppo had blocked a road leading towards the Turkish border that rebels use as a supply route.

The army also took villages including Bashkuwi and Sifat, while fighting raged in Hardatain and Ratain, said the Observatory, which tracks the Syrian conflict using sources on the ground. It added that at least 18 insurgents were killed.

Aleppo is Syria’s second city and is at the forefront of clashes between pro-government forces and a range of insurgents, including Islamist brigades, Al-Qaeda’s hardline Syria wing the Al-Nusra Front and Western-backed units.

A UN mediator has been struggling to broker a ceasefire in Aleppo for months and government forces had long been expected to try to encircle it completely, aiming to drive away rebels.

“It is very important, because if they continue like this they will completely cut the supply lines for the future. The Syrian regime is moving forward there,” the Observatory’s founder Rami Abdulrahman said.

More than 210,000 people have died in the Syrian conflict, which will enter its fifth year next month, the Observatory has estimated, with clashes between numerous factions regularly flaring across the country.

Fighting in southern Syria has killed scores of combatants in the past week in one of the last areas where mainstream rebels opposing President Bashar Al-Assad have a foothold.

In Aleppo province, villages and towns have passed between rival combatants while insurgent-held districts in the divided city have come under heavy air force bombardment.

Syrian state television said on Tuesday five people had been killed and 18 wounded in Aleppo by insurgent rocket fire. The Observatory reported battles between pro-government fighters, the Al-Nusra Front and Islamist battalions in at least six neighborhoods, including Old Aleppo and Jamiat Al-Zahra.

Al-Manar, a television channel run by Lebanese group Hezbollah, said the Syrian army had taken control of areas north of Aleppo in battles which had killed “tens of militants.”

UN envoy Staffan de Mistura has been working since October on a plan to negotiate “local freezes” in Syria, starting in Aleppo. But army progress since then has reduced the chances of a truce, diplomats say.