Government forces have retaken control of a strategically important town in southern Syria, activists say.
Sheikh Miskeen, which lies on one of the main routes from Damascus to the city of Deraa and the Jordanian border, fell after a month-long battle.
Russian warplanes were reported to have played a key role in the offensive.
Russia's foreign minister meanwhile declared its intervention had changed the course of the conflict in Syria, ahead of the start of peace talks.
Sergei Lavrov also warned it would be impossible to negotiate a political settlement without allowing Kurdish groups to attend.
The UN has invited Syria's government and opposition to indirect "proximity talks" that are due to start in Geneva on Friday, but a statement released on Tuesday did not give any details as to who had been invited or how many groups may take part.
There was a delay in sending out the invitations because of disagreements over who should be included in the opposition delegation.
Government soldiers and allied fighters, including members of Lebanon's Hezbollah movement, took control of Sheikh Miskeen overnight, the Syrian Observatory for Human Rights reported.
But fighting was continuing on the western outskirts of the town on Tuesday, the UK-based monitoring group said.
"The town is very important for both sides. They have both fought fiercely. Now by taking it, the regime has cut off the rebels links between eastern and western Deraa [province]," its director Rami Abdul Rahman, told the Reuters news agency. "The destruction in the town is huge."
The region is the last where secular and nationalist rebel factions still hold substantial territory.
The government's offensive in Sheikh Miskeen was the first to be launched in the south following the start of Russia's air campaign against opponents of President Bashar al-Assad on 30 September.
On Tuesday, Russia's foreign minister told a news conference in Moscow that its intervention in Syria had "really helped to turn around the situation in the country" and "helped towards reducing the territory controlled by terrorists".
Mr Lavrov also stressed that no-one had supplied proof to support widespread allegations that Russian air strikes had caused civilian deaths in Syria.
Moscow says it is targeting "all terrorists", above all members of the so-called Islamic State group (IS), but activists say many of its strikes have hit civilians and Western-backed rebels.
The Syrian Observatory, which relies on a network of sources on the ground, said last week that the Russian air campaign had killed 1,015 civilians, as well as 1,141 rebel fighters and 893 Islamic State (IS) militants.
Mr Lavrov also said it would be a "grave mistake" to accede to a demand by Turkey not to invite the Syrian Kurdish Democratic Union Party (PYD) to the upcoming peace talks.
The Kurdish party's YPG militia controls large parts of northern Syria and is a key ally of the US-led coalition against IS.
The PYD is an offshoot of the Kurdistan Workers' Party (PKK), which Turkey and a number of Western countries consider a terrorist organisation.
Mr Lavrov also denied that Russia's military intelligence chief had travelled to Damascus in an attempt to persuade President Assad to step down.
More than 250,000 people have been killed since the uprising against Mr Assad erupted in March 2011. Eleven million others have been driven from their homes.