Jerusalem  - Israel repeatedly fired white phosphorus shells over densely populated areas of Gaza during its offensive there in late December to mid January, a human rights group charged Wednesday.

The firing "was indiscriminate and is evidence of war crimes," Human Rights Watch (HRW) said in its 71-page report.

The organization said that after the offensive ended on January 18, its researchers in Gaza "found spent shells, canister liners, and dozens of burnt felt wedges containing white phosphorus on city streets, apartment roofs, residential courtyards, and at a United Nations school."

White phosphorous, used by armies to provide a smokescreen, ignites and burns on contact with oxygen, and continues burning at up to 1500 degrees Fahrenheit (816 degrees Celsius) until nothing is left or the oxygen supply is cut. When white phosphorus comes into contact with skin it creates intense and persistent burns.

The Israeli army had an alternative, non-lethal method of providing a smokescreen, in smoke shells produced by an Israeli company, HRW said.

"In Gaza, the Israeli military didn't just use white phosphorus in open areas as a screen for its troops," said Fred Abrahams, senior HRW emergencies researcher for Human Rights Watch and the co-author of the report, entitled "Rain of Fire: Israel's Unlawful Use of White Phosphorus in Gaza."

He charged that the Israeli army also "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 was unable to give the number of Palestinians killed or injured by the shells, saying "this was difficult to estimate", but the report analyses six cases in which 12 Palestinians were killed, and dozens injured.

The organization added that it had heard of more cases, however. "We know that it was used extensively and the problem is that it was used extensively in populated areas, that's the key point, because its use in open areas is legal," Abrahams said.

According to HRW, the Israeli military has not yet provided a response to detailed questions submitted on February 1, saying that an internal inquiry was being conducted.

The organization called for an international investigation into "serious laws of war violations by all parties" and said the US, which supplies the shells to Israel, should also conduct an investigation to determine whether they were used "in violation of the laws of war."

The Israeli military did not provide an immediate response to the HRW charges.

Israel launched its Operation Cast Lead on December 27, in response to repeated Palestinian rocket fire on southern Israel, and ended it on January 18.

During the campaign, which began with a week of heavy air strikes before the ground troops entered the enclave, some 1417 Palestinians were killed, and over 5,000 were wounded, the Palestinian Centre for Human Rights said. (dpa)