Israel has denied allegations of illegal use of white phosphorus rounds
Israel's use of white phosphorus shells over densely populated areas during the recent Gaza conflict may constitute war crimes, a rights group has said.
Human Rights Watch accuses Israel of "deliberately or recklessly" using the shells in violation of the laws of war, causing "needless civilian deaths".
The New York-based group's report is based on research conducted immediately after the conflict ended in January.
Israel has insisted that its use of weapons in the offensive was lawful.
It has also said repeatedly that it sought to avoid unnecessary civilian deaths among Gaza's Palestinian population.
White phosphorous, which is used to lay smokescreens, is legal for use on open ground but its use in built-up areas where civilians are found is banned under international conventions.
The Israeli military says it is investigating its use of munitions containing the substance, and the probe is "close to conclusion".
"Based on the findings at this stage, it is already possible to conclude that the IDF's use of smoke shells was in accordance with international law. The claim that smoke shells were used indiscriminately, or to threaten the civilian population, is baseless," it said in a written statement.
Human Rights Watch (HRW) on Wednesday described Israel's repeated firing of white phosphorus shells as indiscriminate and evidence of war crimes.
The group's 71-page report, Rain of fire: Israel's unlawful use of white phosphorus in Gaza, is based on witness accounts of use of the munitions, as well as ballistics evidence and official Israeli documents.
It concludes that the Israeli military "repeatedly exploded white phosphorus munitions in the air over populated areas, killing and injuring civilians and damaging civilian structures, including a school, a market, a humanitarian aid warehouse and a hospital".
HRW researchers found spent shells and canisters containing white phosphorus on streets, apartment roofs and at a UN-run school, report says. All the shells found were manufactured in the US, it adds.
Fred Abrahams, co-author of the report, said: "In Gaza, the Israeli military didn't just use white phosphorus in open areas as a screen for its troops.
"It fired white phosphorus repeatedly over densely populated areas, even when its troops weren't in the area and safer smoke shells were available.
"As a result, civilians needlessly suffered and died."
HRW says senior commanders should be held to account for it describes as "a pattern or policy of phosphorus use" by the military that must have required their approval.
The Israeli military statement also said 155mm smoke shells containing white phosphorous are "used by many Western armies", and said that "contrary to claims in the [HRW] report, smoke shells are not an incendiary weapon".
Earlier this week, UN human rights investigators also questioned the legality of Israel's Gaza offensive in a wide-ranging report to the UN Human Rights Council.
The Israeli organisation Physicians for Human Rights also released a report saying Israel had violated international law and ethics codes during the Gaza operation.
It accused Israeli forces of "attacks on medical personnel, damage to medical facilities and indiscriminate attacks on civilians not involved in the fighting".
The Israeli military says it is investigating specific claims of abuses and argues that it did its utmost to protect civilians during a conflict in which militants operated from populated civilian areas.
The stated aim of Israel's three-week-long offensive was to curb rocket and mortar fire by militants from Gaza.
Palestinian medical authorities say more than 1,300 Palestinians died. Thirteen Israelis, including three civilians, were killed.