In early May, Israeli air strikes in Gaza and Palestinian rocket fire killed 19 civilians in unlawful attacks, Human Rights Watch said.
Four Israeli air strikes last month struck targets with no military objective and caused disproportionate civilian loss, in breach of the laws of war, the group said.
Civilians killed included a pregnant Palestinian woman, an infant, and a boy, 11. Fifteen Palestinian civilians were killed in those strikes.
Unguided rockets launched by Palestinian armed groups towards Israeli population centres were also unlawfully indiscriminate, the organisation said. They claimed four lives.
“Nearly every new round of fighting in Gaza leaves civilians dead and wounded,” said Tom Porteous, deputy programme director at the group.
“Both Israeli and Palestinian armed groups are carrying out attacks with disturbing disregard for the protection of civilians.”
The organisation interviewed 20 survivors or witnesses to attacks, relatives of those killed, or residents of the areas hit.
It also visited the sites of strikes, inspected remnants of munitions, and reviewed statements by Israeli officials and Palestinian armed groups.
Meanwhile, the Israeli rights group B’Tselem also documented the civilian toll from Gaza air strikes by Israel in May.
Fighting broke out on May 3 when Israeli forces fired on unarmed Palestinian demonstrators, killing two, who were gathered near the fences between Gaza and Israel.
Palestinian groups retaliated with gunfire, wounding two Israeli soldiers.
Israeli forces then struck a Hamas post, killing two fighters, triggering barrages of rockets into Israel.
The Israeli military then retaliated with a three-day aerial bombardment campaign.
The military said it struck "350 Hamas and terror targets" between May 4 and 6, but it also hit as many as 30 homes. The strikes killed 25 Palestinians and injured 153.
Human Rights Watch found that four May 5 air strikes killed 13 civilians – two people sitting in a cafe, a teacher at a tutoring centre and 10 people in their homes.
The Israeli military also killed 12 Palestinian fighters, including those from Hamas and another militant group.
But activist groups also documented 33 homes destroyed in Gaza, 327 people left homeless and 700 homes partially damaged.
In two of the four Israeli attacks investigated, Human Rights Watch found no evidence of a military objective, such as enemy fighters or weapons or other military material, needed for an attack to be legal under the laws of war.
“There is little hope for reining in attacks that violate the laws of war so long as both Israeli and Palestinian forces can commit them with impunity,” Mr Porteous said.
“The continued unlawful killing of civilians underlines the need for a formal ICC probe in Palestine to help bring a measure of justice for the victims and their families.”