Syrian troops backed by Hezbollah fighters have reportedly launched a major offensive in the country's south even as the UN envoy holds talks with President Bashar al-Assad aimed at "reducing" the violence.

State media and activists said on Wednesday that Syrian government forces launched a fresh offensive near the Israeli-occupied Golan Heights aimed at recapturing areas seized by opposition fighters.

Syrian state TV said the military gained control of the town of Deir al-Adas and the village of Deir Maker, as well as the nearby areas of Tal al-Arous and Tal al-Sarjeh.

The Syrian Observatory for Human Rights, which monitors the conflict through activists on the ground, said Syrian troops were "advancing in the area linking Deraa, Quneitra and Damascus provinces".

"The operations are being led by Hezbollah's special forces," Rami Abdurrahman, who heads the Britain-based activists' network, said.

"Their aim appears to be to eventually reach areas bordering the occupied Golan and set up a border zone under Hezbollah's control."

Hezbollah, an Iran-backed Shia group, has offered crucial military support to the Assad government during the nearly four-year civil war that has left hundreds of thousands dead and millions displaced.

'Freeze' in fighting

In the capital, Damascus, Staffan de Mistura, the UN peace envoy, met Assad to discuss his proposal for reducing the violence, starting with a hostilities freeze in the northern city of Aleppo.

"My focus has been and is the importance of reducing the violence and conflict for the Syrian people, [and] the increase and unimpeded access of humanitarian assistance to all Syrians," De Mistura said.

He did not disclose details of the meeting, but state-run news agency SANA said the two discussed "new details" in the UN plan to freeze fighting in Aleppo "in a positive and constructive climate".

In October, de Mistura put forward an " action plan " for Syria proposing to "freeze" fighting in local areas to allow for aid deliveries and to lay the groundwork for peace talks.

He has frequently said that Aleppo, Syria's commercial hub before the war, would be a "good candidate" for such a "freeze".

The increasingly fragmented conflict raging in Syria has claimed more than 210,000 lives since erupting in March 2011.