Russian Foreign Minister Sergei Lavrov says he has submitted a proposal for a ceasefire in Syria, as world powers meet in Munich.
Mr Lavrov gave no further details about the plan, but it is understood to envisage a truce starting on 1 March.
The US is demanding an immediate end to hostilities, as it suspects Russia wants to give Syrian government troops three weeks to crush rebel forces.
Russian air strikes are helping the Syrian army advance in Aleppo province.
The UN High Commissioner for Human Rights, Zeid Raad Al Hussein, wants the world powers to ensure talks in Geneva aimed at finding a political solution to the conflict are reconvened as soon as possible, following their suspension last week.
“The warring parties in Syria are constantly sinking to new depths, without apparently caring in the slightest about the death and destruction they are wreaking across the country,” he said. “It is a grotesque situation.”
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 (ISSG) in Munich will bring together 17 countries, including both allies and opponents of Mr Assad.
Mr Lavrov and US Secretary of State John Kerry met each other before the talks to discuss what the Russian foreign minister described as a “quite specific” ceasefire proposal.
“We will wait for the American response before we take it to the ISSG,” he added.
Moscow has proposed that the ceasefire start on 1 March, but Washington fears that rebel forces might suffer irreversible losses in northern and southern Syria before then, US officials say.
Mr Kerry wants a ceasefire to be effective immediately and accompanied by humanitarian organisations being given full access to the 400,000 people trapped in besieged areas.
The UN human rights chief earlier 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.
Russia’s defence ministry said it had carried out almost 1,900 air strikes in Syria over the past week.
But it denied that it had bombed two hospitals in Aleppo on Wednesday, stressing that only US aircraft had flown over the city that day. The US military dismissed the claim, calling it “patently false”.
Turkish President Recep Tayyip Erdogan used a live television address to warn on Thursday that refugee numbers could increase by 600,000 if the Russian strikes continue.
Criticising 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.
The UN Office for the Co-ordination for Humanitarian Affairs separately warnedthat 120,000 people were trapped in a rebel-held rural area of northern Homs province.
Several cases of acute malnutrition among pregnant women and children had been reported, as well as deaths related to lack of medical care, the OCHA said.
Activists meanwhile confirmed Kurdish fighters had captured the Menagh airbase, north of Aleppo, after driving out Islamist rebels from the facility overnight. The UK-based Syrian Observatory for Human Rights reported that the Kurdish advance was backed by Russian air strikes.
Saudi state television also quoted a military spokesman as saying the kingdom’s offer to send ground troops to Syria was an “irreversible decision” and Defence Minister Prince Mohammed bin Salman vowed to increase its contribution to the US-led campaign against IS.