The Syrian army, backed by Russian airstrikes, pressed ahead with a major attack in the northern province of Aleppo on Friday, forcing tens of thousands of civilians to flee to the Turkish border.
The Syrian Observatory for Human Rights said that regime troops and fighters from the Lebanese Hezbollah movement and Iranian Revolutionary Guards retook control of the town of Ratyan on the northern outskirts of Aleppo from rebels early on Friday.
Hours later, the rebels launched a counter-attack and regained more than half of Ratyan, the Britain-based Observatory said.
More than 100 fighters from both sides have been killed in Ratyan so far, according to the watchdog.
The ongoing fighting in the area has forced an exodus of some 40,000 people towards the Turkish border, which is still closed by the Turkish authorities, according to activists.
The UN estimated that by late Friday some 30,000 people had gathered at the Turkish-Syrian border after fleeing the violence.
Videos posted on social media have shown people lining roads near the northern Syrian-Turkish border.
Turkey is reportedly sending aid to the displaced people on the Syrian side of the border, including tents and food supplies, the pro-government Yeni Safak newspaper said.
Syrian regime forces and their allies have in the past two days cut off the main supply route from Aleppo to Turkey, a key backer of Syria's opposition.
"Taking Ratyan will enable the regime forces to fully control roads around the towns of Nubul and Zahraa, which they recaptured two days ago," the head of the Observatory, Rami Abdel-Rahman, told dpa.
Rebels had seized Ratyan more than a year ago.
On Wednesday, forces loyal to Syrian President Bashar al-Assad broke rebels' long siege of Nubul and Zahraa, marking a significant victory for the regime, which has tried for more than two years to encircle Aleppo, the last major urban centre in rebel hands.
Once Syria's commercial hub, Aleppo has been split between rebel control in the east and regime control in the west since shortly after fighting began in the province in mid 2012.
In yet another setback for rebels, regime forces Friday regained the strategic town of Ataman in the southern province of Daraa, state media and the Observatory reported.
Ataman is strategically important because it is located only 2 kilometres from Daraa city, the capital of the province of the same name.
The regime, backed by Russian jets, started a major attack against rebels in Daraa province in December.
Daraa, which borders Jordan, is a stronghold of moderate rebels.
The Syrian army has made territorial gains in the war-torn country since ally Russia began an air campaign in September.
In New York, Francois Delattre, French ambassador to the UN, said that UN Syria envoy Staffan de Mistura confirmed to the UN Security Council that the Geneva peace talks had been suspended because of the Syrian government's military offensive with support from Russia.
"The Syrian regime and its allies have made no concession, quite the contrary," Delattre said.
"The special envoy's decision to suspend the talks was the only possible decision under the current circumstances."
Vitaly Churkin, Russian ambassador to the UN, said the French ambassador's characterization "put words into de Mistura's mouth," noting that the special envoy merely expressed hope for "a better environment for the talks" and made no reference to Russian airstrikes.
He said Russia was planning to "bring some new ideas to the table" to move forward with the peace process during the upcoming meeting of the International Syria Support Group in Munich next week.
US Secretary of State John Kerry said the Russians had made proposals as part of ceasefire talks but "if it’s just talk for the sake of talk in order to continue the bombing, nobody’s going to accept that. And we will know that in the course of the next days."
He accused both Russia and Syria of violating a UN resolution passed late last year calling for access for humanitarian aid and an end to bombing of civilians in Syria.
Kerry faulted Russia for using so-called free-fall or dumb bombs in its Syrian bombing campaign, in violation of the UN resolution Moscow supported, and said women and children were among those being killed as a result.
"This has to stop," he said.