The main pro-Russian rebel leader in eastern Ukraine says his troops are on the offensive and he does not want truce talks with Kiev.
Alexander Zakharchenko said his forces would push the front line back to the borders of Donetsk region. They are in control of the city of Donetsk.
Russian President Vladimir Putin accused Ukrainian government forces of “criminal” shelling of civilian areas.
Kiev “ordered the start of major combat operations” along the front, he said.
Earlier, Mr Zakharchenko said the rebels “will not make any attempts at ceasefire talks any more”.
Ukraine, Russia, France and Germany have issued a call to end fighting.
On Thursday, Ukrainian government troops withdrew from Donetsk airport’s main terminal, a scene of bitter fighting in recent weeks.
The government said the military still retained control of parts of the airport, but six soldiers had died and 16 had been wounded.
The government and its Western allies say Russian regular troops are fighting alongside the separatists, using Russian heavy artillery and tanks. Moscow insists that only Russian “volunteers” have joined the rebels.
Mr Zakharchenko said “we’ll attack right up to the borders of Donetsk region, but if I see a threat from other directions we’ll neutralise it”.
“Kiev doesn’t understand now that we can attack in three directions simultaneously,” Russian media quoted him as saying.
More than 5,000 people have been killed in fighting since the rebels seized a large swathe of Donetsk and Luhansk regions last April, UN officials say. More than a million people have been displaced.
A rebel statement on Friday said Ukrainian forces’ shelling of Donetsk and Horlivka had killed 16 civilians in the past 24 hours.
The OSCE security organisation has confirmed that at least eight civilians died in a mortar blast at a bus stop in Donetsk. The rebels and the government traded blame for the attack, which left more than a dozen wounded.
“It is another crime against humanity, an obvious provocation aimed at undermining the peaceful political process,” Russian Foreign Minister Sergei Lavrov said.
A Ukrainian military spokesman, Andriy Lysenko, blamed a rebel mortar team, saying the explosion was “beyond the range of Ukrainian artillery of this type”.
In Moscow, President Putin told the Russian Security Council that Ukraine was using heavy artillery, rockets and aircraft “indiscriminately against densely-populated areas”.
“The ones responsible for that are those who give such criminal orders,” he said, quoted by Interfax news agency.
He said Kiev had not responded coherently to his proposal for a withdrawal of heavy weapons from the conflict zone.
Rebel military spokesman Eduard Basurin said 24 rebel troops had been killed and 30 wounded in the latest fighting. He called it “the heaviest losses in our ranks” in a 24-hour period.
The Ukrainian Defence Council chief, Oleksandr Turchynov, confirmed on Friday that the rebels had launched an offensive. “The enemy will stop at nothing,” he said, accusing the rebels and Russian forces of intensive shelling.
“Russian terrorist groups have essentially violated all prior ceasefire agreements… and are today assuming active offensive operations,” he said.
“We are talking about active units of the Russian armed forces,” he stressed. According to Kiev, 9,000 Russian troops are fighting alongside the rebels.
The fighting raged on just hours after Ukraine, Russia, France and Germany had issued a joint call for a ceasefire.
They also agreed on a line of demarcation between separatists and government forces from which both sides were meant to withdraw their forces.