A US push for an immediate ceasefire in Syria has run into Russian opposition ahead of talks aimed at reviving stalled peace negotiations.
Russia is reportedly calling for a halt to hostilities on 1 March, but the US suspects it wants to give the Syrian army three weeks to crush rebel forces.
Russian air strikes have been helping the army make major gains in Aleppo province, displacing 50,000 people.
World powers will discuss the ceasefire proposal at a meeting in Munich later.
The UN High Commissioner for Human Rights, Zeid Raad Al Hussein, called on them to ensure that talks in Geneva aimed at finding a political solution to the conflict resume as soon as possible.
The talks were suspended last week while still in the preparatory phase, with both sides blaming each other for the failure to make any progress.
More than 250,000 people have died in almost five years of war in Syria. Eleven million others have fled their homes as forces loyal to President Bashar al-Assad and those opposed to his rule battle each other, as well as jihadist militants from so-called Islamic State (IS).
Thursday’s meeting of the International Syria Support Group in Munich will bring together both allies and opponents of Mr Assad, including Russia, Iran, the US and Saudi Arabia.
Before they began, Russian Deputy Foreign Minister Gennady Gatilov said Moscow was “ready to discuss the modalities of a ceasefire”, without giving any further details.
US officials say Russia has proposed that the ceasefire start on 1 March.
One official told the Associated Press that the US could not accept that date because rebel forces might suffer irreversible losses in northern and southern Syria before the ceasefire took hold.
US Secretary of State John Kerry wants a ceasefire to be effective immediately and accompanied by humanitarian organisations being given full access to besieged areas, where the UN says more than 400,000 people are living in dire conditions.
France’s permanent representative to the UN, Francois Delattre, said: “The [Syrian] regime and its allies cannot pretend they are extending a hand to the opposition while with their other hand they are trying to destroy them.”
The UN human rights chief also on Thursday expressed “utmost alarm” at the rapidly worsening human rights situation in and around the city of Aleppo.
Since the offensive by government forces began on 1 February, dozens of civilians had been killed and some 51,000 displaced, Mr Zeid said, adding that 300,000 others in rebel-held eastern Aleppo were at risk of being placed under siege.
“We have also received numerous reports of destruction of civilian infrastructure, including at least three clinics and two bakeries since the launch of this latest round of hostilities,” he added.
The US military meanwhile described as “patently false” Russian claims that US aircraft bombed Aleppo on Wednesday. The US earlier alleged that Russian air strikes had destroyed two clinics in Aleppo – something Russian officials denied.
Russian ambassador Vitaly Churkin also said his country was not about to be “apologetic” for its actions in Syria, accusing other UN Security Council members of exploiting the humanitarian situation for their own political gain.
Turkish President Recep Tayyip Erdogan used a live television address to warn on Thursday that refugee numbers could increase to 600,000 if the strikes continue.
Attacking the mounting pressure on Turkey to allow in 30,000 of those displaced by the fighting around Aleppo who are stranded at its border, he said Turkey would be patient up to a point, but then would be forced to take action.
Nato member states later agreed to begin naval patrols in the Aegean Sea to deter people smugglers plying their trade between Turkey – which is hosting more than 2.5 million Syrian refugees – and Greece.