The United States urged Russia on Friday to use its influence on the Syrian regime to stop heavy fighting in northern Syria that has overshadowed peace talks in Geneva.
US Secretary of State John Kerry told his Russian counterpart that the US expected Russia to urge the regime to comply with the a cessation of hostilities, US State Department spokesman John Kirby said.
He said Kerry told Russian Foreign Minister Sergei Lavrov during a phone call Friday that the US would work with the opposition to do the same. Kerry also expressed "serious concerns" over ongoing threats to the ceasefire and the urgent need to stop the violations.
With heavy backing from Russian warplanes, government forces have waged a large-scale offensive on the northern city of Aleppo since Thursday, trying to advance on rebel-held strategic areas there, activists said.
The latest battles in Aleppo are seen as the biggest threat to the partial ceasefire, which has been in force in Syria since February.
The attack aims to cut off a major supply route for the rebels, Mahmoud al-Shami, an activist based in the city, said.
"As usual before every round of negotiations the regime is sending a strong message that it doesn't want a political solution," chief opposition negotiator Asaad al-Zoubi said in Geneva.
The government of President Bashar al-Assad is "seeking a military solution that will bring destruction to the whole country," he added.
At the same time, the opposition struck a conciliatory tone when it said it would accept a transitional governing body that could include some officials of the current administration, in line with demands by the UN Security Council.
"So we will work with the other side, including with diplomats and technocrats, but not with Assad or those who have committed crimes against Syrians," opposition delegation spokesman Salem al-Meslet told dpa.
Should al-Assad's forces take control of the northern edge of Aleppo, they would impose a siege on rebel-controlled areas there, the head of the Syrian Observatory for Human Rights, Rami Abdel-Rahman, told dpa.
Aleppo, Syria's pre-war commercial hub, has been divided between regime forces based in the west and rebels to the east since fighting erupted for the control of the city in mid-2012.
The partial ceasefire was brokered by the United States and Russia excludes Islamic State and Syria's al-Qaeda branch, al-Nusra Front.
Elsewhere in northern Syria, regime forces were Friday battling Islamic State militants in the area of Khanasser south-east of Aleppo city, the Observatory and Syria's state news agency SANA reported.
Islamic State's advances in northern Aleppo in the past few days have forced 60,000 people living in camps east of the town of Azaz to flee, New York-based Human Rights Watch said.
The watchdog called on Turkey to reopen its border to the refugees.
The surge in violence came as al-Assad's negotiating team arrived for the latest round of indirect talks in Geneva that started on Wednesday.
Bashar Jaafari, the government's chief delegate, said his side had handed proposals to UN Syria envoy Staffan de Mistura on amending his list of topics on which the regime and rebels already see eye to eye.
De Mistura's list includes political transition; Syria's territorial unity; a national effort to defeat terrorism; the rejection of foreign fighters; and the return of refugees.
"We held a session of dialogue and talks with special envoy [de Mistura] which I can say was constructive and fruitful," Jaafari said, without giving any details.
The government and opposition delegations have not yet met face to face at the negotiating table. Instead, de Mistura has been shuttling between both sides.