Kiev's pro-Western leaders hope to hold truce talks on Saturday with pro-Russian separatists despite the rebels' vow to push their latest offensive in eastern Ukraine if the negotiations should fail.
The urgent new round of negotiations in Minsk that had been agreed for Friday under pressure from European envoys was postponed due to disagreements over who should represent the rebel camp.
Kiev said it expected to send its envoy, former Ukrainian president Leonid Kuchma, to Minsk on Saturday for the talks -- formally backed by the Kremlin -- aimed at reinforcing a tattered September truce.
"We expect to sign a document that reinforces the Minsk Memorandum (of September) and the peace plan of presidents (Petro) Poroshenko and (Vladimir) Putin," Kuchma told the Interfax-Ukraine news agency.
"Our main goal is to ensure that the (September) agreement is implemented," he told Ukrainian reporters earlier of the talks mediated by European and Russian envoys.
Plans for the negotiations in the Belarussian capital Minsk were announced on Thursday, raising hopes of dialogue after the collapse of a September truce in the nine-month war that has killed more than 5,100 people, according to the United Nations.
Ukraine is insisting on the presence of Donetsk insurgency commander Alexander Zakharchenko and leader of the separatist Lugansk region Igor Plotnitsky at the talks, rather than their representatives, a Ukrainian diplomatic spokesman told AFP.
The insurgents last week pulled out of peace talks and announced the start of an offensive designed to expand their control over a much broader swathe of the industrial southeast.
They also said Friday they would not halt their actions in restive areas if the talks failed.
"Should the negotiations collapse... the Donetsk and Lugansk People's Republics reserve the right to pursue their offensive until the entire Donetsk and Lugansk regions are freed" of Ukrainian troops, the rebel regions' main negotiators said in a joint statement.
- Tattered truce -
The insurgents' statement said the fighters were ready to pull back their heavy weapons from the frontline as long as Ukrainian forces did the same.
But they also stressed that the new border outlining rebel-run regions should run along the current front, giving them an area around 500 square kilometres (200 square miles) greater than lines agreed in September.
The latest violence has alarmed Ukraine's Western allies, with US Secretary of State John Kerry announcing plans to express his support for the war-torn nation during talks in Kiev on Thursday with Poroshenko and Prime Minister Arseniy Yatsenyuk.
Meanwhile on Friday Poroshenko reaffirmed that an "immediate ceasefire" was necessary in a phonecall with German Chancellor Angela Merkel.
Local officials and the Kiev military said 19 civilians and five Ukrainian soldiers died in the latest wave of clashes in Ukraine's eastern industrial heartland since Thursday afternoon.
A mortar strike on an aid distribution point in the self-proclaimed rebel capital Donetsk killed five civilians.
"A member of my family died here. He brought people in his car so they could collect humanitarian aid. The shell hit his car, he was torn apart," Vera, 56, told AFP at the scene.
- Key town 'surrounded' -
Heavy fighting was also reported around Debaltseve -- a key government-held town of 25,000 people that was built around a railroad connecting the two rebel centres of the Russian-speaking southeast.
Zakharchenko told Russian state television that Ukrainian troops in the town were "surrounded" and unable to receive supplies or send their wounded for treatment in regional hospitals.
Western governments and Ukraine accuse Russia of arming and training the rebels, who are deploying extensive sophisticated and heavy weaponry, including tanks and multiple rocket launchers. Russia denies aiding the rebels who claim to get all their weaponry from captured Ukrainian supplies.
The 28-nation EU on Thursday extended through September a first wave of targeted sanctions it had slapped on Moscow and Crimean leaders in the wake of Russia's March seizure of the Black Sea peninsula from Ukraine.
EU foreign ministers also agreed to start work on further "appropriate action" if Moscow and the rebels continued breaching the original terms of the collapsed September truce.
Russia accuses the West of manipulating the Ukrainian government, which came to power in elections after the ouster in huge street demonstrations last year of a Kremlin-backed leader.
The Kiev government has angered Moscow by seeking closer ties with the EU and future membership in NATO, which would bring the Western military alliance into a huge section of the former Soviet Union.