A US push for an immediate ceasefire in Syria has run into Russian opposition ahead of talks aimed at reviving peace negotiations on the conflict.
Russia proposed a 1 March ceasefire, but the US believes Moscow is giving itself and the Syrian regime three weeks to crush rebels, reports quoting Western officials said.
World powers are meeting in Munich later for talks on the Syria conflict.
Discussions come amid fresh warnings of a worsening humanitarian situation.
A surge in fighting in Syria’s Aleppo province has displaced about 50,000 people, the International Committee of the Red Cross (ICRC) warned – adding that supply routes for aid had been cut, putting civilians under “enormous pressure”.
And reports quoting the Russia defence ministry revealed on Thursday that Russian planes carried out 510 military sorties in Syria in the last week.
Russia says its strikes target what it calls terrorists, but Western powers say they have been used against mainstream opposition groups fighting the Syrian government.
‘Trying to destroy opposition’
US Secretary of State John Kerry is expected to push for an immediate ceasefire and access for aid workers in Aleppo when Russia, the US, Saudi Arabia, Iran and other powers meet in Munich later.
But Syrian officials have indicated no plans to ease up the battle to return Aleppo to state control.
And Russian ambassador Vitaly Churkin said his country was not about to be “apologetic” for its actions, accusing other Security Council members of exploiting the humanitarian situation for their own political gain.
Western officials suggest Washington has concerns about Russia’s ceasefire proposal and that no agreement has been reached.
Meanwhile French UN Ambassador Francois Delattre said on Wednesday night: “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.”
Medecins Sans Frontieres (MSF) has warned that fighting in Azaz, near the Turkish border, has left health systems “close to collapse”.
New arrivals from the fighting have been forced to seek shelter in already thinly-stretched refugee camps, according to the charity.
Turkey is under pressure to open its border to an estimated 30,000 people fleeing the Aleppo fighting.
It has already taken in more than 2.5 million Syrian refugees over the past five years and says it will continue to do so in a “controlled fashion”.
Up to 300,000 could be cut off from aid if the offensive by Syrian government forces and Iranian-backed militias encircles rebel-held eastern Aleppo, the UN has said.