Fighting has claimed more than 20 lives in eastern Ukraine as the leaders of Russia, Ukraine, France and Germany gather for peace talks.
Shelling killed five people in rebel-held Donetsk city on Wednesday while 19 Ukrainian soldiers died on Tuesday.
The talks in the Belarusian capital Minsk are set to focus on securing a ceasefire, withdrawal of heavy weapons and creating a demilitarised zone.
Berlin and Paris have been working hard on a deal for the past week.
French President Francois Hollande and German Chancellor Angela Merkel were due to meet Ukrainian President Petro Poroshenko before full talks with Russian President Vladimir Putin.
Speaking after his arrival, Mr Poroshenko said the situation threatened to spin "out of control" if the parties did not agree to de-escalation and a ceasefire.
While the Ukrainian government is seeking a demarcation line based on the failed ceasefire agreed on 5 September in Belarus last year, the rebels want a new truce to reflect the gains they have made in recent weeks, the BBC's James Reynolds reports from Donetsk.
More than 5,400 people have died since the conflict began, and there has been a dramatic rise in casualties in recent days, with 263 civilians killed in populated areas between 31 January and 5 February.
Ceasefire hopes: James Reynolds, BBC News, Donetsk
Donetsk may be the tidiest city-at-war in the world. In the centre of town, the streets are swept, the benches are clean, the bins are empty. From the city centre you can hear the regular, deep boom of artillery fire in the distance.
"We want to live alone," insists one woman who puts down her shopping bags to talk. "Most people here don't want to be ruled by Ukraine and [President] Poroshenko," she concludes.
"We're hoping for a resolution," says another man, "But it all depends on Ukraine."
A mother, walking with her young son, says she's exasperated with both sides. "They need to shut all the leaders in a room," she suggests, "like they do when they choose a Pope. Lock them in. Don't let any of them out until they've reached an agreement."
With both sides seeking territorial gains before a potential ceasefire, there are fears that the violence could undermine attempts to secure a long-term deal.
The majority of Ukrainian military casualties were near Debaltseve, a major transport hub where thousands of soldiers are under rebel siege.
The separatists say they have cut off the main supply road into the town from the west as they try to capture a tongue-shaped area that cuts into the eastern regions of Donetsk and Luhansk.
At least two people were killed and several more injured when a shell hit a bus station in rebel-controlled Donetsk early on Wednesday. Three more deaths were reported during the night, city officials said.
Volunteer pro-government fighters have also seized territory this week, capturing villages from the rebels outside the government-held of Mariupol.
Rival agendas at Ukraine talks
Ukraine: Restore government authority over breakaway areas, though Donetsk and Luhansk regions could get greater self-rule; disarm rebel forces; withdrawal of Russian troops; restore Kiev's control over Ukraine-Russia border; full prisoner exchange.
Pro-Russian rebels: Separation from rest of Ukraine and recognition of "people's republics" of Donetsk and Luhansk; no disarmament of separatist forces; amnesty for separatist leaders.
Russia: Legal guarantees for rights of Russian-speakers in eastern Ukraine; full autonomy for Donetsk and Luhansk in a federal system - not necessarily independence; no return of Crimea to Ukraine; withdrawal of Ukrainian troops from combat zone.
EU and US: Restore Ukraine's territorial integrity; end Russian intervention in eastern Ukraine - withdrawal of all Russian troops and heavy weapons; effective monitoring of Russia-Ukraine border and demilitarised zone between the combatants; full democracy in Donetsk and Luhansk.
Ukraine's President Petro Poroshenko said Wednesday's summit was one of the final chances to bring about an unconditional ceasefire and withdrawal of heavy weapons.
But he warned that his government was prepared to impose martial law throughout Ukraine if the conflict escalated further.
EU foreign policy chief Federica Mogherini told AFP news agency that if a deal was reached, the Minsk summit would be "a turning point for good or for bad".
Ukraine and the West accuse Russia of sending troops and arms to support the rebels, but Russia denies this.
US President Barack Obama has refused to rule out supplying "lethal defensive weapons" to Kiev if diplomacy fails, but Russia says that would worsen the crisis.
Ukraine's war: The human cost
- At least 5,486 people people killed and 12,972 wounded in eastern Ukraine but true numbers could be much higher
- Fatalities include 298 people on Malaysia Airlines flight MH17 shot down on 17 July
- 5.2 million people estimated to be living in conflict areas
- 978,482 internally displaced people, including 119,832 children
- 600,000 fled to neighbouring countries, of whom more than 400,000 have gone to Russia
Source: Figures from UN report, 6 February