Ukrainian volunteers carry a victim out of the City Hall building, following shelling in Kharkiv. AP

Most of the civilian casualties were caused by explosive weapons with a wide impact area, including shelling from heavy artillery and multi-launch rocket systems, and from missile and air strikes, monitors from the Office of the UN High Commissioner for Human Rights (OHCHR) said.

"OHCHR believes that the real figures are considerably higher, especially in Government-controlled territory and especially in recent days, as the receipt of information from some locations where intensive hostilities have been going on was delayed and many reports were still pending corroboration," it said.

The mission said hundreds of civilian casualties alleged in the town of Volnovakha — where attempts were under way to open a safe evacuation corridor through encircling Russian forces - were yet to be corroborated.

Reuters