A new round of peace talks to end Syria's five-year civil war is set to begin in Geneva on Wednesday, as a surge in violence in northern Aleppo province threatened a fragile truce.
The United Nations-brokered talks are aimed at forming a transitional government followed by general elections to end a conflict that has killed more than 270,000 people and displaced half of the country's population.
The fate of President Bashar al-Assad remains a stumbling block, and UN Syria envoy Staffan de Mistura has held meetings with Assad's key allies Tehran and Moscow ahead of a sit-down with the main opposition High Negotiations Committee Wednesday and regime representatives later in the week.
A surge in violence in the last few days has threatened a landmark ceasefire agreed in February -- which had largely held -- and piled more pressure on these talks, which follow two previous fruitless attempts to negotiate an end to the bloodshed.
"Right now, there are signs that this (the ceasefire) is slipping and it is a much more delicate environment for de Mistura to convene political talks," US Ambassador to the UN Samantha Power told reporters in New York after a briefing by de Mistura on Tuesday.
Power said Russia must put pressure on its Syrian ally to "get the regime back with the programme", adding she was "very alarmed" by Syria's plans to launch a Russian-backed counter-offensive in Aleppo, the epicentre of the renewed fighting.
Iran's Deputy Foreign Minister Hossein Amir Abdollahian also voiced concerns about the increase in fighting following de Mistura's trip to Tehran, saying it was "disturbing and may interfere with the political process".
Another point of tension as the indirect talks resume, are parliamentary elections being held Wednesday in government-controlled areas of Syria.
The UN does not recognise the vote, which has also been dismissed by Assad's Syrian and foreign opponents as illegitimate.
Polling stations opened at 7:00 am (0400 GMT) in areas under government control -- about a third of the country's territory where about 60 percent of the population lives.
The election is expected to see Assad's Baath party maintain control over parliament, although several parties are participating.
The partial truce brokered by Moscow and Washington, which came into effect on February 27, had raised hopes for a resolution to the conflict, until the recent escalation in fighting in northern Aleppo province, in parts of Hama province and Damascus.
Pro-government forces were on Tuesday pressing an advance against the town of Al-Eis, held by Al-Qaeda's local affiliate, Al-Nusra Front, and allied rebels, the Syrian Observatory for Human Rights said.
Al-Nusra and the rebels fought back, killing at least 23 regime loyalists, including Iranian and Afghan militia fighters, the Observatory said.
Jihadists like those from Al-Nusra and the Islamic State group are excluded from the ceasefire. But in some areas, Al-Nusra is allied with rebel forces meant to be covered by the truce.
Intra-jihadist fighting also raged in southern Damascus, where IS on Tuesday took control of most of a Palestinian refugee camp, seizing territory held by Al-Nusra, a Palestinian official told AFP.
Regime warplanes have carried out "unprecedented" air strikes on the rebel-held eastern parts of Aleppo city, according to the Observatory.
"I didn't send my child to school today because I was afraid of more air strikes like in the past two days," said Ismail, a 30-year-old Aleppo resident.
Abu Mohammad fled his home in Aleppo during the peak of the Syrian air force's bombing campaign but came back when the situation improved. He now fears his return may be short-lived.
"I am preparing myself to leave the city should the bombing continue," the 38-year-old father of four told AFP.
Washington on Monday expressed worries that an assault against Al-Nusra in Aleppo may spread to moderate rebel factions, which could cause the truce to collapse and derail the peace efforts.
"We are concerned about plans to attack and seize... Aleppo when there are clearly opposition groups there that are part of the cessation of hostilities," State Department spokesman Mark Toner told reporters.
The truce has allowed increased humanitarian aid deliveries and a significant drop in civilian deaths.
"The ceasefire has been so important over the last weeks because it has given people a lot more than simply access to markets, access to assistance. It has given them hope," the World Food Programme's deputy regional head Matthew Hollingworth told AFP in Damascus.