Pro-Russian rebels in Ukraine's east have called for a ceasefire in Donetsk to avert a "humanitarian catastrophe".
Newly installed rebel political leader Aleksandr Zakharchenko made the call after other rebels said the army had taken a key city and encircled Donetsk.
Russia immediately renewed its offer to send a humanitarian mission, a day after Western powers warned against using such a mission to send troops.
An estimated 1,500 people have been killed in the four-month conflict.
Pro-Russian rebels stormed cities in the east and took over government buildings in April in a bid for independence.
But the government stepped up operations to retake rebel-held areas following the election of Petro Poroshenko as president in June.
Rebel commander Igor Girkin was quoted by Russian media as saying that Krasnyi Luch, which connects Donetsk city with Ukraine's Luhansk region, had been "captured".
Girkin, who is also known as Strelkov, said his men in the Donetsk region were "completely encircled".
Mr Zakharchenko later said in a statement on a rebel website: "We are prepared to stop firing to bar the spread of the scale of the humanitarian catastrophe."
Russia, widely accused of supporting the rebels' efforts, also called for an "urgent action to avert an impending humanitarian crisis".
Foreign Minister Sergei Lavrov made the request during a phone call with US counterpart John Kerry, a statement said.
"The minister urged support for Russia's initiative to deploy a humanitarian mission in the south-east in co-ordination with the relevant international structures," the foreign ministry statement said.
Mr Kerry apparently replied that such efforts were already being made by the Ukrainian government.
On Friday, the UK and US had warned Russia not to use a humanitarian mission as a pretext for sending its forces to help the rebels.
Meanwhile, reports from the city of Luhansk, second only to Donetsk in its importance to the rebels, suggest living conditions are dire.
The city council reported on its website (in Russian) on Saturday that the city of 425,000 people had been without electricity and power for a week.
Parts of the city were still being bombarded and most shops were shut.
The Ukrainian government, which has not commented on rebel claims about the situation in the east, says 13 soldiers and one civilian were killed in fighting on Friday.
In other developments
- The Russian navy "expelled" a US submarine from its territorial waters in the Barents Sea on Thursday, a navy source told Russian media
- Russia announced it was freeing five Ukrainian officers for "humanitarian reasons", a day after announcing their arrest for suspected war crimes in Ukraine
- The authorities in the Ukrainian capital Kiev dismantled protest barriers erected during the winter in the city centre