The firing of rockets at Israel by Palestinians in Gaza during the conflict in May amounted to war crimes, Human Rights Watch (HRW) has said.

The attacks "flagrantly violated" the laws of war, the campaign group said following an investigation.

More than 4,360 unguided rockets and mortars were fired, Israel says, killing 13 people there. Some fell short, killing Palestinians in Gaza.

At least 260 people were killed in Gaza during the 11 days of fighting.

It began after weeks of spiralling Israeli-Palestinian tension in East Jerusalem which culminated in clashes at a holy site revered by both Muslims and Jews. Hamas - the militant Islamist group which rules Gaza - began firing rockets after warning Israel to withdraw from the site, triggering retaliatory air strikes.

Last month, HRW said three Israeli air strikes it investigated also amounted to war crimes. The investigation into the strikes which killed 62 civilians found no evidence of military targets nearby.

The Israeli military says it only struck military targets in Gaza and took various measures to avoid causing civilian casualties.

In its report released on Thursday, HRW said: "The rockets and mortars that Palestinian armed groups fired lack guidance systems and are prone to misfire, making them extremely inaccurate and thus inherently indiscriminate when directed toward areas with civilians. Launching such rockets to attack civilian areas is a war crime.

"Hamas authorities should stop trying to justify unlawful rocket attacks that indiscriminately kill and injure civilians by pointing to Israel's violations," Eric Goldstein, HRW acting Middle East and North Africa director, said. "The laws of war are meant to protect all civilians from harm."

The report says Palestinian rockets and mortars which misfired killed and injured an undetermined number of people in Gaza. It said one misfire above the city of Jabalya which it investigated killed seven civilians and injured 15. According to the Israeli military, 680 rockets and mortars fired by Hamas fell inside the Gaza Strip.

The Begin-Sadat Center for Strategic Studies, an Israeli think-tank, has estimated the misfires killed 91 people.

HRW says it is preparing to release a report on Israeli air strikes that destroyed or damaged four high-rise towers in Gaza.

Hamas and Israel have been in a state of conflict and have fought several wars since the group won Palestinian elections in 2006 and reinforced its power in Gaza the following year when it ousted the rival Fatah faction of Palestinian President Mahmoud Abbas.