Syrian government forces are reported to have captured two strategic villages in the south-west of the country.
State media said troops and militiamen had seized Busra al-Hariri and Malihat al-Atash in Deraa province, cutting a strip of rebel-held territory in half.
The government launched an offensive in the region, which borders Jordan and the Israel-occupied Golan Heights, a week ago despite warnings from the US.
The UN says at least 45,000 people have already been displaced by the fighting.
The region had been relatively calm in the past year because of a “de-escalation” agreement brokered by the US and Jordan, which support the opposition, and Russia, a key ally of the government.
However, President Bashar al-Assad has set his sights on regaining control of the south-west since defeating rebels in the Eastern Ghouta region outside Damascus.
Syria’s state news agency, Sana, cited a military source as saying that the army and allied militias seized Busra al-Hariri and Malihat al-Atash after inflicting heavy losses on “terrorists” from al-Nusra Front, a former al-Qaeda affiliate now known as Hayat Tahrir al-Sham.
Hundreds of square kilometres had now been recaptured in the past few days in the al-Lujat area of north-eastern Deraa, Sana reported.
There was no immediate comment from rebel sources, but the Syrian Observatory for Human Rights confirmed that the government had taken the two villages and cut off a rebel-held area to the north from territory extending to the Jordanian border.
The UK-based monitoring group said at least 15 rebels were killed in the battle, meaning that a total of 29 rebels and 24 pro-government fighters had been killed since troops stepped up air and artillery strikes last Tuesday.
At least 32 civilians, including children, are also reported to have lost their lives.
UN officials said on Tuesday that another 45,000 people had fled the fighting and that they expected the figure to more than double if the violence escalated. The region is home to an estimated 750,000 people.
Many of those displaced are heading towards the border with Jordan, but its government has warned that it will be unable to host a new wave of refugees because of the strain they would place on its financial resources and infrastructure.
It has already taken in 666,000 refugees registered with the UN since the Syrian civil war began in 2011, but it has said the actual number of Syrians in Jordan is closer to 1.3 million.
Jordan has joined the US in calling for the “de-escalation” agreement covering Deraa and neighbouring Quneitra province to be preserved in order to prevent a wider confrontation between Israel and Iran. Washington has warned the government of “repercussions”, but told rebel commanders that it would not intervene to protect them.
Iran is an ally of Mr Assad and has deployed hundreds of troops to Syria. Thousands of militiamen armed, trained and financed by Iran – mostly from Lebanon’s Hezbollah movement, but also Iraq, Afghanistan and Yemen – have also been fighting alongside the Syrian army.
Israel has vowed to stop what it considers Iran’s military “entrenchment” in Syria and has carried out dozens of missiles strikes on Iranian military and militia positions, as well as weapons shipments.
On Tuesday, the Sana news agency reported that two Israeli missiles hit an area near Damascus international airport overnight.
It gave no further details, but the Syrian Observatory for Human Rights reported that the Israeli missiles had struck a Hezbollah arms depot.
There was no immediate comment from the Israeli military, which usually refuses to confirm or deny such reports.
Syria war: Government forces make gains in south-west}