Human Rights Watch said in a statement that neither side has carried out an impartial investigation.
"Israeli investigations still fall far short of being thorough and impartial, while Hamas appears to have done nothing at all to investigate alleged violations," the group said.
Four Israeli prosecutions
The investigations are a response to the so-called "Goldstone report," which was written by a panel led by Richard Goldstone, a South African judge. It accused both sides of war crimes, but most of its criticism was directed at Israel, which killed roughly 1,400 people during the war, according to rights groups.
The UN in November gave both sides three months to investigate the report. It extended that deadline by another five months in February.
Israel's update to the UN said the Israeli army launched more than 150 investigations into alleged crimes committed during the war. Roughly one-third of those resulted in criminal investigations, and four of those investigations led to prosecutions.
Two officers were indicted for using artillery too close to civilian homes, for example.
But the Israeli report also excused some of the most serious acts of destruction documented in the Goldstone report. The report accused Israel of violating international law, for example, by destroying Gaza's only working flour mill and a chicken coop housing more than 100,000 birds.
Israeli army investigations concluded that neither of those incidents violated the law.
The Goldstone report called for the UN to refer its findings to the International Criminal Court if Israel and Hamas do not carry out proper investigations.