Eastern Ukraine's Donetsk hit by fresh fighting
There has been heavy fighting in the eastern Ukrainian city of Donetsk as government forces upped operations against Russian-backed militants.
A government spokesman said Ukrainian forces had retaken most of the area around the bitterly contested airport.
A Kremlin spokesman said Russia was "concerned" by the escalation.
In Ukraine's capital, Kiev, thousands attended a rally for 13 civilians who died in the east when their bus came under rocket attack last Tuesday.
Addressing the rally, President Petro Poroshenko paid tribute to those defending Donetsk airport from rebels, saying they had "demonstrated their courage, patriotism, heroism, as a model for how our country must be defended".
"We will not give away one scrap of Ukrainian land," he told the crowd.
Donetsk airport no longer functions but has taken on symbolic value for both sides.
Around Donetsk, Ukrainian government officials said there had been massive shelling of separatist positions as the army launched a counter-attack.
"The decision was taken for a mass operation," military spokesman Andriy Lysenko said on TV.
"We succeeded in almost completely cleaning the territory of the airport, which belongs to the territory of Ukrainian forces as marked by military separation lines."
Locals said there had been intense fighting, including in residential areas, and several civilians were reported to have been killed.
Rebel leader Alexander Zakharchenko accused Kiev of "trying to unleash war again".
A spokesman for Russian President Vladimir Putin, Dmitry Peskov, said Moscow was concerned by the "escalation of hostilities", adding that it undermined a ceasefire agreed in Minsk in September that has been repeatedly violated by both sides.
"This state of affairs in no way contributes to the implementation of the Minsk agreements and future search for a resolution," Mr Peskov said.
Ukraine says some 8,500 Russian regular troops are helping the rebels.
More than 4,700 people have been killed since the rebels took control of a big swathe of Donetsk and Luhansk regions in eastern Ukraine last April. Many more civilians have been displaced by the fighting.
Russia denies sending regular troops and heavy weapons there, but admits that Russian "volunteers" are helping the rebels.