Syria conflict: Aleppo fighting threatens peace talks
The UN is to start a new round of indirect talks in Geneva aimed at ending the conflict in Syria.
UN special envoy Staffan de Mistura has warned that there is an urgent need for the government and opposition to take steps towards a political transition.
But correspondents say the outlook is bleak, with fighting in Aleppo province threatening a fragile six-week truce.
Meanwhile parliamentary polls, dismissed by the opposition as a sham, are under way in government-held areas.
Russia, a staunch ally of President Bashar al-Assad, has said the vote does not go against the peace talks and is in line with the constitution.
But the opposition and its backers say the elections are illegitimate and provocative.
Some 3,500 candidates are standing in the polls, which are being held in about a third of the country's territory where about 60% of the population lives.
Aleppo offensive 'alarming'
Members of the main Syrian opposition umbrella group, the High Negotiations Committee (HNC), arrived in Geneva on Tuesday ahead of the start of the third round of "proximity" talks since January.
Government representatives have said they will not join them until Friday.
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After meeting Iranian Deputy Foreign Minister Hossein Amir-Abdollahian in Tehran on Tuesday, Mr de Mistura said he wanted this round to be "quite concrete".
It was now "crucially urgent", he added, that both sides agreed on a political process that a UN Security Council resolution passed in December envisages will lead to the formation of a transitional government, a new constitution and elections.
The veteran Swedish-Italian diplomat also stressed that it was very important that the cessation of hostilities brokered by the US and Russia continued and that humanitarian aid was allowed to reach every Syrian.
The US permanent representative to the UN, Samantha Power, later warned that the escalation of violence in Aleppo province threatened to derail the talks.
Samantha Power said the US, which backs the opposition, was "very alarmed" at the government's announcement of a major offensive south of the city of Aleppo and that Russian needed to "get the regime back with the programme".
The Syrian Observatory for Human Rights (SOHR), a UK-based monitoring group, said dozens of pro-government fighters were killed on Tuesday as they attempted to retake the village of al-Eis.
The government offensive is being supported by Russian air strikes, Iranian Revolutionary Guards personnel and fighters from Lebanon's Hezbollah movement.
Al-Eis, which is located near the Damascus-Aleppo motorway, was recently captured by al-Nusra Front, an al-Qaeda affiliate that is excluded from the cessation of hostilities but allied to rebel groups that are included.
Government warplanes also carried out "unprecedented" air strikes on rebel-held eastern parts of Aleppo city on Tuesday, according to the SOHR.
Ms Power also criticised the government for allowing only two humanitarian convoys in April and no access to the besieged Damascus suburb of Darayya where "there are reports of kids walking around looking like skeletons" and people eating grass.
Meanwhile, the UN World Food Programme said another successful airdrop had been conducted on Tuesday over government-held areas of the eastern city of Deir al-Zour, where more than 200,000 people are under siege by jihadist militants from so-called Islamic State (IS).