The United Nations' human rights office says the number of people killed in more than a year of fighting in eastern Ukraine has risen to over 6,400.

The office said Monday that at least 6,417 people have been killed and 15,962 wounded between April last year and Saturday.

The latest numbers compare with figures of 6,116 dead and 15,474 wounded given in mid-April. Shelling diminished following a February cease-fire deal, but fighting has worsened in recent weeks.

The U.N. high commissioner for human rights, Zeid Raad al-Hussein, said there are "alarming reports of summary executions by armed groups" and his office is "looking into similar allegations against Ukrainian armed forces." He added that there are "horrific accounts of torture and ill-treatment in detention" by both sides.