Fierce fighting raged Friday around Syria's Aleppo as a surge in violence forced tens of thousands more to flee their homes, overshadowing the latest round of peace talks in Geneva.
The clashes on several fronts have put a strain on a fragile ceasefire in place since February 27, and left more than 200 fighters on all sides of the civil war dead in the past week.
US Secretary of State John Kerry urged Russia to press its Syrian allies to respect the ceasefire.
The delegation representing President Bashar al-Assad's regime arrived Friday in Geneva for UN-brokered indirect talks between representatives of the government and opposition.
Syria's main opposition High Negotiations Committee (HNC) told AFP it was willing to join a transitional government with diplomats and technocrats from Assad's government.
"We cannot accept the participation of the parties who committed crimes against the Syrian people in the transitional governing body," HNC spokesman Salem al-Meslet said on the sidelines of the talks.
But the opposition could cooperate with regime "diplomats and technocrats" provided they had popular support, he said.
The Geneva talks resumed following legislative elections in regime-held areas on Wednesday.
UN envoy Staffan de Mistura met the government delegation, with a second session set for Monday, and the HNC was holding its second meeting with him on Friday.
Lead government negotiator Bashar al-Jafaari described his meeting as "constructive and fruitful".
- 'Major ceasefire violation' -
Assad's role in a future transitional government, which de Mistura has said would be the focus of the talks, remains the key sticking point.
Damascus says that even discussing Assad's departure is off limits, while the opposition insists he can have no role in a future transition.
The fighting around Syria's second city Aleppo has cast a shadow over international efforts to end the five-year war, which has left more than 270,000 people dead and forced millions to flee their homes.
Troops and militiamen loyal to the regime have fought Islamic State group fighters southeast of Aleppo city this week, the Syrian Observatory for Human Rights said.
They also battled jihadists from the Al-Qaeda-affiliated Al-Nusra Front and allied rebels in the flashpoint Handarat area, north of Aleppo city, it said.
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"What is happening in Aleppo is a major violation of the ceasefire," rebel commander Major Eyad Shamsi told AFP in Geneva, blaming the regime.
"A big battle is being fought in Aleppo, and it will lead to a major disaster should the regime succeed" in severing the route linking rebel-held parts of Aleppo to the northern countryside, he said.
Kerry called Russian Foreign Minister Sergei Lavrov to say Washington "expected Russia to urge the regime to comply with the cessation and that we would work with the opposition to do the same", US spokesman John Kirby said.
On one Aleppo front alone fighting has forced about 30,000 civilians to flee, according to Human Rights Watch.
- Aleppo 'the key' -
HRW accused Turkish border guards of shooting at some of those displaced as they approached the frontier.
"As civilians flee ISIS fighters, Turkey is responding with live ammunition instead of compassion," said HRW researcher Gerry Simpson, using another acronym for the jihadists.
At least 210 fighters on all sides have been killed around Aleppo since Sunday.
Among them were 82 soldiers and pro-regime militiamen, 94 members of Al-Nusra Front and allied rebel groups, and 34 IS jihadists, the Observatory said.
Even though IS and Al-Nusra are excluded from the truce, violence around Aleppo has sparked concerns that the ceasefire may collapse, partly because rebels are involved there too.
"Aleppo is the key to war and peace in Syria," Observatory director Rami Abdel Rahman told AFP.
"Every side in the war has a stake in Aleppo."
The fighting around the city is the fiercest in Syria since the truce began nearly seven weeks ago, and is especially significant because all sides in the war are present in the province.
Russia, which has been supporting regime forces with air strikes, blamed Al-Nusra for the escalation.
"According to the data we have, southwest of Aleppo around 8,000 Al-Nusra fighters are already gathered, and north of the city up to 1,500," a Russian foreign ministry statement said.
"The actions of the Syrian troops supported by the Russian air force are aimed at wrecking the plans of the Al-Nusra bandit groups," it said, stressing that "no storming of Aleppo is planned".
Syria's conflict began in 2011 as a peaceful revolt but later morphed into a multi-front civil war.