A Ukrainian military spokesman on Sunday rejected a pro-Russian separatist call for a cease-fire, as fighting continued unabated in eastern Ukraine.
A day earlier, the separatists said they were ready for a cease-fire to prevent a "humanitarian disaster" in eastern Ukraine.
But on Sunday, Ukrainian military spokesman Andriy Lysenko rejected the appeal and called on the rebels to "raise white flags" and lay down their weapons.
Ukrainian troops have surrounded Donetsk in a "tight ring" and the only thing for rebels left to do is surrender, said Lysenko, the Ukrainian National Security and Defense Council spokesperson.
The separatists themselves backed away from their appeal, saying on Sunday that any cease-fire must be "mutual," and that the Ukrainian government is incapable of reaching an agreement and therefore any discussion of a cease-fire is "pointless."
The Ukrainian military continued to squeeze separatist forces in their remaining strongholds of Donetsk and Luhansk. Residents of Donetsk reported heavy shelling Sunday, and heavy fighting was also reported in Luhansk.
In Kievsky district, 6 kilometers west of Donetsk's city center, black smoke was rising above headquarters of Ukrainian telecommunication company “Ukrtelecom."
A nearby apartment block was damaged as well, with many windows shattered and balconies destroyed.
Elsewhere, Russian Foreign Minister Sergei Lavrov said on Sunday Moscow was in talks with Kyiv, the International Committee of the Red Cross and the United Nations on sending humanitarian aid to eastern Ukraine.
“We think it is a priority now to reach an agreement with the Ukrainian side, International Committee of the Red Cross and the international U.N. humanitarian agencies on the necessity to send emergency humanitarian aid to the (eastern Ukrainian provinces) of Luhansk and Donetsk,” Lavrov told reporters.
According to the U.S. State Department, Secretary of State John Kerry told Lavrov in a phone conversation Saturday that the best way to get humanitarian aid into Ukraine is through international organizations that are already on the ground.
Kerry told Lavrov that Russia should not intervene in Ukraine under the pretext of peacekeeping or humanitarian convoys.
As of last week, Russia had amassed about 20,000 troops just across the border.
The White House said President Barack Obama and German Chancellor Angela Merkel agreed by phone Saturday that any Russian intervention in Ukraine without Kyiv's consent would violate international law and bring additional consequences.
The Ukrainian government has steadily retaken territory from the insurgents over the last few weeks, ousting them from smaller provincial cities and encircling Donetsk.
Commanders said the army has been making steady gains and has virtually encircled the separatists' second-largest stronghold of Luhansk, while rebels have declared a “state of siege” in Donetsk, the largest city they hold.
Kyiv security officials said separatist fighters were launching counter-attacks to break a tightening noose around the rebels who seek to set up pro-Russian "people's republics" in the east of Ukraine.
A storm of Donetsk by crack troops appears to have been ruled out: Ukraine has little or no expertise in such missions and it would risk heavy civilian casualties and army losses.
A siege is the most likely option, with the possibility of an escape corridor for the separatist fighters in the hope they may flee to Russia.
Conditions were clearly deteriorating in Donetsk, the largest rebel stronghold in eastern Ukraine.
AP reporters heard 25 loud explosions in as many minutes around noon on Sunday. More than 10 residential buildings, as well as a hospital and a shop, were heavily damaged by shelling overnight, and several buses caught in the crossfire were still burning Sunday morning.
Donetsk city council spokesman Maxim Rovinsky told AP Saturday that over 2,000 residential buildings have been damaged by shelling.
At least 300,000 of Donetsk's 1 million residents have fled as violence escalates between Ukrainian government forces and pro-Russian separatists, claiming the lives of over 1,300 people since April, according to the United Nations.
Some information for this report provided by Reuters and AP.