Syria's opposition has rejected a proposal from the UN envoy that would have kept Bashar al-Assad as president during a political transition, with three deputies of his opponents' choosing.

UN envoy Staffan de Mistura told Syria's opposition attending peace talks in Geneva that the proposal could end the "vicious cycle" of debate over a transitional period to end the war, a source told AFP on Saturday.

On the ground, tens of thousands of Syrians are at risk of being displaced as clashes between rebels and militants intensified in the country's north.

The escalating fighting across swathes of Aleppo province has threatened to collapse a fragile ceasefire and derail the latest round of indirect negotiations in Switzerland between the regime and opposition.

The fate of Assad remains the key sticking point in the discussions involving the opposition High Negotiations Committee and a government delegation.

The HNC insists Assad must leave, but Damascus objects to that demand.

A HNC source told AFP on Saturday that the committee had rejected a proposal by de Mistura that would have seen Assad remain as president through a transitional period.

In exchange, the HNC would have been allowed to hand-pick three vice presidents, the source said on condition of anonymity because he was not authorised to speak to the press.

Assad "would transfer his military and political prerogatives to them. Effectively, Assad would stay in a ceremonial position."

"But we categorically rejected the proposal," he said.

Assad's ouster has been the key demand of Syria's opposition since the uprising broke out in March 2011, but Damascus says his departure is not on the table.

While the opposition insists on forming a "transitional governing body" without Assad, the regime says it wants to form a broader "unity government."

The HNC source said de Mistura had presented the idea of Assad transferring most of his powers to three deputy presidents from the opposition as a way to end that "vicious cycle" of debate.

According to him, de Mistura told the opposition that the proposal "was not his personal view... but that he hoped to hear our thoughts".

On Friday, HNC spokesman Salem al-Meslet told AFP that Syria's opposition would be willing to cooperate with regime "diplomats and technocrats" in a transitional period.

But he insisted that there would be no role for Assad or anyone who had played a central role in the civil war, which has killed 270,000 people and displaced millions.

A militant onslaught on opposition-held territory in northern Syria has displaced about 30,000 civilians in recent days and left thousands of others near the Turkish border at risk, Human Rights Watch said.

"We're talking about more than 100,000 people now sandwiched between Aleppo and the border and affected by this crisis," HRW's Gerry Simpson told AFP by phone from the Turkish side of the frontier.

Those Syrians are already living in displacements camps scattered near the border "and are perilously close to the shifting front lines," Simpson said.

He said people were sleeping under open skies without access to food or sanitation.

Turkey has kept its border closed through recent waves of displacement, a decision that Simpson called "unconscionable".

The new scramble for safety comes as IS group presses a fierce offensive against non-militant rebel groups in Aleppo province, seizing another village on Saturday, according to the Syrian Observatory for Human Rights.

Rebels had previously advanced from Azaz, a town eight kilometres (five miles) south of the Turkish border, towards IS bastions further east.

But IS has pushed back with a fierce counter-attack.

"The IS gains have cut off opposition territory around Azaz from rebels in the town of Dudyan further east," said Observatory head Rami Abdel Rahman.

"Now the rebels in Dudyan are practically surrounded by IS."

The militants also scored gains against regime forces near Khanasser, a battleground town southeast of Aleppo that has changed hands several times.

The road through Khanasser is the sole link between government-held areas in and around Aleppo and those in the rest of the country.

And in Aleppo city itself, two civilians were killed by rebel fire on a residential neighbourhood in the government-held west of the city, the Observatory said.

The city was once Syria's commercial hub, but since rebels seized eastern districts in 2012 a frontline cuts through the heart of the city.

The only remaining road out of opposition-controlled neighbourhoods was under threat this week from a fierce Russian-backed government offensive.

Fighting has surged on several fronts throughout Aleppo province, which is criss-crossed with supply routes that are strategic for practically all of Syria's warring sides.

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