There has been an intense artillery bombardment overnight in the eastern Ukrainian city of Donetsk, reports GHN based on BBC.
A BBC correspondent in the city said the shelling was the worst in more than a month, lasting almost eight hours.
She said it appeared to be coming from both rebel and government positions.
The fighting comes a day after large numbers of unmarked military vehicles were seen in rebel-held areas. Ukraine says it believes the rebels have received equipment from Russia.
More than 4,000 people have died since fighting erupted in April after pro-Russian separatists seized control in the regions of Donetsk and Luhansk.
A fragile ceasefire has been in place since 5 September, although hundreds of people have been killed since then.
Fears of a return to full-scale conflict were raised after the separatists held elections on Sunday in defiance of Ukraine's government and Western countries.
Russia has recognised the vote which led to separatist leader Alexander Zakharchenko, 38, being sworn in as head of the self-declared Donetsk People's Republic.
Igor Plotnitsky, a 50-year-old ex-Soviet army officer, was declared head of the self-proclaimed Luhansk People's Republic, further to the east.