Syrian government forces have captured new areas from insurgents in their offensive in the north-west, as Turkey sends more reinforcements into the country, state media and opposition activists said.

The weeks-long government offensive has created a humanitarian crisis with about 600,000 people having fled their homes in Syria’s last rebel stronghold since the beginning of December, according to the United Nations.

Rebels control much of Idlib province and parts of the neighbouring Aleppo region that is home to 3 million people, many of them displaced from other parts of Syria.

The offensive appears aimed at securing a strategic highway in rebel-controlled territory, as opposed to an all-out campaign to retake the entire province, including the densely populated provincial capital, Idlib.

“Our aim is to clear the highway and evict terrorists from it,” a Syrian commander on the ground told state TV. He was referring to the M5 highway, which links the state capital, Damascus, with the northern city of Aleppo.

The Britain-based Syrian Observatory for Human Rights, an opposition war monitor, said government forces still had 18 miles (30km) of the highway to clear before it came under full control of the army for the first time since 2012.

Syrian state TV reported on Saturday that government forces captured four villages in Aleppo province near the highway. It added that Syrian troops and demining experts had cleared explosives from the recently captured town of Saraqeb which sits on an intersection of the M5 and M4 highways, linking Syria’s coast with the country’s east.

Turkey, a main backer of the opposition, sent more reinforcements into Idlib overnight, according to the Observatory and the Idlib-based media activist Taher al-Omar who is embedded with militants.

The Observatory said a convoy of 430 vehicles had entered Syria since Friday night, raising the number of vehicles to have entered the country since last weekend to well over 1,000.

A Turkish military convoy reaches north-east Idlib on Friday night. Photograph: Yahya Nemah/EPA

A rare clash between Turkish troops and Syrian soldiers earlier in the week left seven Turkish soldiers and a Turkish civilian dead as well as 13 Syrian troops.

On Friday, Turkey’s defence ministry said the army would respond “even more forcefully” to any attack on Turkish observation posts in the area.

The violence has raised tensions between Russia and Turkey, which have been working together to secure ceasefires and political talks despite backing opposite sides of the conflict.