At least four civilians, including a pregnant woman, were killed in explosions at a village checkpoint in a rebel-controlled territory in eastern Ukraine on Wednesday, said the Organisation for Security and Cooperation in Europe (OSCE).
The Donetsk People's Republic (DPR or DNR), an unrecognised state and a pro-Russian rebel group based in the Donetsk region of Ukraine, also said that at least 15 people were injured as they crossed between government- and rebel-controlled areas.
Ukraine's Army denied carrying out any attacks at that time in the territory.
OSCE said it had been recording "a high number of ceasefire violations" in the region alone, listing hundreds of blasts and gunfire heard in both government and rebel-held areas in recent days.
Fighting between the Ukrainian Army and pro-Russian rebels broke out in April 2014, after the Russian annexation of Crimea, as separatists took control of most parts of the Donetsk and Luhansk regions of eastern Ukraine.
The conflict has increased sharply despite calls to observe a year-old peace agreement, the OSCE said.
Both sides of the conflict accuse each other of igniting the violence.
According to a UN report, at least 9,000 people have been killed and 21,000 injured in the conflict, while 2 million others have been forced to flee their homes.
Twenty Ukrainian troops were reported killed in March, which was the army's highest death toll in a month since August. So far in April, 17 Ukrainian soldiers have been killed.