Up to 20,000 Syrians were stranded on the Turkish border Friday after fleeing a major Russian-backed regime offensive near Aleppo where a new humanitarian disaster appeared to be unfolding.

Tens of thousands of civilians have joined an exodus to escape fierce fighting involving government forces who severed the rebels' main supply route into Syria's second city.

On Friday, clashes between the two sides in and around Ratyan, a town near Aleppo, cost 120 lives, said the Syrian Observatory for Human Rights.

The UN's Office for the Coordination of Humanitarian Affairs (OCHA) said it estimated "up to 20,000 people have gathered at the Bab al-Salama border crossing and another 5,000 to 10,000 people have been displaced to Azaz city" nearby.

Western nations have accused the Syrian government of sabotaging peace talks that collapsed this week with its military offensive, and Washington has demanded Moscow halt its campaign in support of President Bashar al-Assad.

The UN Security Council met later Friday to discuss the faltering peace process, as NATO head Jens Stoltenberg warned Russian air strikes were "undermining the efforts to find a political solution", a charge dismissed by Moscow.

The Syrian Observatory, a Britain-based monitor that relies on a network of sources on the ground, estimates that 40,000 people have fled the regime offensive near Aleppo.

"Thousands of people, mainly families with women and children, are waiting to enter Turkey," Observatory director Rami Abdel Rahman told AFP.

OCHA spokeswoman Linda Tom said that in addition to the thousands at the border, another 10,000 people were estimated to have been displaced to the Kurdish town of Afrin, elsewhere in northern Aleppo.

- 'Rebels on the retreat' -

"The fighting has also disrupted major aid and supply routes from the Turkish border," she said.

Aleppo province is one of the main strongholds of Syria's opposition, which is facing possibly its worst moment since the country's brutal conflict began in 2011.

"The trajectory for the rebels is downwards, and the downward slope is increasingly steep," said Emile Hokayem, a senior fellow at the International Institute for Strategic Studies.

"The rebels are on the retreat everywhere."

Turkish Prime Minister Ahmet Davutoglu said Thursday up to 70,000 people were heading to his country, which already hosts about 2.5 million Syrian refugees.

Early Friday, the main border crossing in northern Aleppo was closed and quiet on the Turkish side near the town of Kilis, with no sign of arriving refugees.

But footage released Thursday by activists showed hundreds of people, including many children, heading towards the frontier, some carrying their belongings in plastic bags on their backs.

More than 260,000 people have died in Syria's conflict and more than half the population has been displaced.

Aleppo city, Syria's former economic powerhouse, has been divided between opposition control in the east and regime control in the west since mid-2012.

Syria's army has been on the offensive since staunch government ally Russia began an aerial campaign in support of regime forces on September 30.

- Regime retakes key towns -

Since then, the regime has recaptured several key rebel towns in Latakia province -- Assad's coastal heartland -- and advanced in Aleppo province and in Daraa in the south.

On Friday, the army seized the town of Ratyan and village of Mayer, north of Aleppo, with support from dozens of Russian air strikes.

But a rebel counteroffensive saw opposition fighters regain half of Ratyan in heavy fighting that killed some 60 rebels and the same number of regime forces, the Observatory said.

Pro-government troops backed by Russian warplanes also retook a rebel bastion in Daraa used as to launch attacks on the provincial capital, the monitor said.

The losses have angered and demoralised Syria's opposition.

"What frustrates the rebels the most is that the countries that claim to be their friends are happy with empty words and sitting on the fence," said activist Maamoun al-Khatib.

"Meanwhile (regime allies) Russia and Iran are occupying and violating Syrian territory."

Top diplomats from countries trying to resolve the conflict are set to meet again on February 11 after UN-brokered peace talks in Geneva collapsed this week.

But tensions between them remain, with Moscow accusing key opposition backer Ankara of actively preparing to invade Syria, a claim Turkish President Recep Tayyip Erdogan dismissed as "laughable".

The Pentagon, meanwhile, welcomed a Saudi pledge to commit ground forces to fight Islamic State jihadists in Syria, should the US-led coalition ever send in combat troops.

In New York, the Security Council met for closed-door consultations with UN envoy Staffan de Mistura who suspended the floundering Geneva negotiations on Wednesday until February 25.

Russia curtly rejected Western accusations that it had sabotaged the peace talks by backing the regime's Aleppo offensive.

"It's in bad taste. This is not a good time for recriminations," Russian ambassador Vitaly Churkin told reporters.

A diplomat said the United States, France and Russia had "animated exchanges" on the topic at the meeting.