A leading human rights group says it has strong evidence Ukraine attacked populated areas of Donetsk with cluster bombs, banned by many other states.
Human Rights Watch (HRW) accuses the army of using the deadly weapon, which scatters bomblets, to shell the rebel-held city earlier this month.
Ukrainian forces denied using "banned weapons" or shelling civilian areas.
Violence has continued in eastern Ukraine despite a ceasefire which came into place on 5 September.
At least 3,700 people have been killed in the conflict which broke out in April between Ukrainian forces and pro-Russian separatist rebels in the eastern regions of Donetsk and Luhansk.
Ukrainian President Petro Poroshenko agreed in a telephone conversation on Tuesday with his Russian counterpart, Vladimir Putin that the ceasefire agreed on 5 September should be fully observed, Mr Poroshenko's office said.
Meanwhile, the Ukrainian government has asked the EU for an extra loan of €2bn (£1.6bn; $2.5bn).
A source at the European Commission in Brussels told BBC News the money was not linked to Ukraine's ongoing gas debt talks with Russia, which are being brokered by the EU.
Russia is accused of using gas as a political tool against Ukraine.
In a separate development, a senior Polish official has alleged that Mr Putin suggested partitioning Ukraine with Poland six years ago in a conversation with Donald Tusk, who was Poland's prime minister at the time.
'Shocking to see'
In its report, the New-York based rights group said use of cluster munitions in Donetsk this month was "widespread".
"While it was not possible to conclusively determine responsibility for many of the attacks, the evidence points to Ukrainian government forces' responsibility for several cluster munition attacks on Donetsk," it found.
"It is shocking to see a weapon that most countries have banned used so extensively in eastern Ukraine," said Mark Hiznay, senior arms researcher at HRW.
Ukraine did not sign up to the 2008 global treaty banning cluster bombs.
Responding to the report, Ukrainian "anti-terrorist operation" spokesman Vladyslav Seleznyov described the accusations as "utter nonsense".
"We do not use banned weapons," he told AFP news agency by telephone. "Neither do we shell civilian neighbourhoods because this endangers lives."
The Convention on Cluster Munitions has been ratified by 86 states while a further 28 have signed it. Russia and the US are among those that have not signed it.
'Just a coincidence'
EU Energy Commissioner Guenther Oettinger has opened a new round of gas talks in Brussels between Ukraine and Russia, amid concerns the gas dispute between Russia and Ukraine could affect supplies to EU states.
A spokesman for the European Commission, Simon O'Connor, confirmed Ukraine had asked for an additional €2bn on top of the €15bn the EU offered in March.
"This will now be evaluated in consultation with the IMF and Ukrainian authorities, and a proposal will be made to the European Parliament and the EU Council," Mr O'Connor said.
A source at the Commission told the BBC that the request was "not related to today's trilateral gas talks", which were "just a coincidence".
Asked what reason Ukraine had given, the source said it was to "cover their most urgent financial needs".
Russia is demanding €4.2bn in unpaid debts, to be repaid in instalments.
Poland's parliamentary speaker, Radoslaw Sikorski, alleged Mr Putin had suggested dividing up Ukraine, in talks in 2008 with Donald Tusk, who will shortly take over as president of the European Council.
"He wanted us to become participants in this partition of Ukraine..." Mr Sikorski told US website Politico. "He [Putin] went on to say Ukraine is an artificial country and that Lwow [Lviv] is a Polish city and why don't we just sort it out together."
"We made it very, very clear to them - we wanted nothing to do with this," Mr Sikorski added.
Speaking later, Mr Sikorski said his words to Politico had been "over-interpreted", without elaborating.
Polish Prime Minister Ewa Kopacz, who is in the same party as Mr Sikorski, criticised him for evading reporters' questions on the issue.
Kremlin spokesman Dmitry Peskov dismissed the reported conversation as a "fairy tale".