Ukraine conflict: Battles rage in Donetsk and Luhansk
Dozens of people have been killed or wounded as fighting escalates along the front line in eastern Ukraine and the battle for Donetsk airport continues.
Artillery fire was reported in several areas of the eastern Donetsk and Luhansk regions and a hospital in Donetsk city was hit, reports said.
Ukrainian forces and pro-Russian rebels both said they controlled the airport.
As Russia warned Ukraine not to make a strategic mistake, Kiev said more Russian troops had crossed the border.
Russia has repeatedly denied claims that its regular forces are in eastern Ukraine, but has admitted that a number of "volunteers" are fighting alongside the rebels.
The surge of violence began last week as the fight for control of the airport intensified. Government forces fought back after rebels said they had finally seized the new terminal building having besieged government forces for months.
The rebel military command said its forces had halted a government offensive at the strategically important Putylivskiy bridge between the airport and the city. The bridge was destroyed in the fighting.
Buildings in the centre of Donetsk have been badly damaged and there are reports that a hospital has been hit. Eight civilians have been killed and many more wounded in recent days, local authorities say.
Meanwhile the rebels accuse government forces of shelling civilians in rebel-held Horlivka, a town north of the airport. Nine civilians were killed there and 44 wounded by a bomb dropped by a Ukrainian plane, said Eduard Basurin at the rebel "defence ministry".
A boy and his father were killed and 10 other civilians were wounded when shells hit the government-held town of Debaltseve, north-east of Donetsk, Ukrainian TV reported.
The escalating fighting prompted Russian Deputy Foreign Minister Gregory Karasin to warn Ukraine that relying on a military solution was "the biggest, even strategic mistake" that could "lead to irreversible consequences for Ukraine's statehood".
Ukraine said two large groups of Russian forces had crossed the border into Ukraine on Monday, but there was no independent confirmation.
Donetsk airport - a valuable prize
Donetsk airport has symbolic and strategic value to both sides. It has been the scene of heavy fighting for months.
Ukrainian media view it as a symbol of "Ukrainian fighting spirit". The troops defending the airport have been called "cyborgs" for their toughness in repulsing constant attacks, and for many they symbolise a new Ukrainian army. Social media users say the destruction of the airport looks like Stalingrad during World War Two.
Separatists have tried hard to undermine the "cyborg myth". They view the airport as part of their capital and, while it remains in government hands, a bridgehead for a potential Ukrainian offensive.
Despite the airport's infrastructure being almost completely destroyed, some experts have pointed out that its runway could still be used for flying in supplies, which is proving difficult for the rebels.
Donetsk airport lay in ruins on Monday after days of fierce fighting.
Military officials said on Sunday they had "succeeded in almost completely cleansing the territory of the airport" but rebel leader Alexander Zakharchenko denied the claims on Monday.
Both sides admitted sustaining losses. A senior rebel officer in Donetsk said eight rebel soldiers had died and 33 were wounded while Ukrainian defence officials said three soldiers had been killed and dozens more were hurt.
A presidential adviser in Kiev said many soldiers in the airport terminal were wounded when a ceiling collapsed after it was blown up by separatists.
Russian officials say the Ukrainian President, Petro Poroshenko, did not respond to a proposal from President Vladimir Putin for the warring sides to pull back their heavy weapons and stop fighting.
However, Ukraine insists that it wants the September ceasefire - agreed in Minsk - to be implemented. The foreign ministry urged Russia again to withdraw troops and heavy weapons from the conflict zone.
Ukraine says some 8,500 Russian regular troops are helping the rebels, a claim denied by Moscow.
More than 4,800 people have been killed since the rebels took control of a parts of Donetsk and Luhansk regions in Ukraine last April. Many more civilians have been displaced.
The fighting began a month after Russia annexed Ukraine's southern Crimea peninsula.