Israeli army had deliberately or recklessly used white phosphorus shells over densely-populated areas of Gaza Strip during its recent military offensive, the New York-based Human Rights Watch (HRW) said in a report released on Wednesday.

The spokesperson of Israel Defense Forces (IDF) responded later that the 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," the spokesperson said in a statement.

In the 71-page report, Rain of Fire: Israel's Unlawful Use of White Phosphorus in Gaza, witnesses recounted the devastating effects that white phosphorus munitions inflicted on civilians and civilian property in Gaza.

HRW researchers found in Gaza immediately after hostilities spent shells, canister liners and dozens of burnt felt wedges containing white phosphorus on city streets, apartment roofs, residential courtyards and at a UN school, the report said.

The report also presents ballistics evidence, photographs, and satellite imagery, as well as documents from the Israeli military and government.

White phosphorus, primarily used to obscure army's operations on the ground by creating thick smoke, ignites on contact with oxygen and continues burning at 816 degrees Celsius until none is left or the oxygen supply is cut. It can also be used as a weapon, producing extreme burns if it makes contact with skin.

When used in open areas, white phosphorus munitions are permissible under international law.

But HRW said Israeli army "unlawfully" fired them over populated neighborhoods in violation of the laws of war.

"In Gaza, the Israeli military didn't just use white phosphorus in open areas as a screen for its troops," said Fred Abrahams, senior emergencies researcher at HRW and co-author of the report. "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."

In the report, HRW called for an investigation into those who ordered or carried out unlawful attacks using white phosphorus munitions.

The US government, which supplied Israel with its white phosphorus munitions, should also conduct an investigation to determine whether Israel used it in violation of the laws of war, HRW said.

HRW submitted in February detailed questions to the IDF about its white phosphorus use in Gaza. The IDF did not respond, citing an internal inquiry being conducted by its Southern Command.