Human Rights Watch said on Thursday that the Israeli army's investigation of troop conduct during its war on Gaza appeared to be an attempt to cover up "violations of the laws of war."

The army defended its conduct during the massive 22-day offensive against the Hamas rulers of Gaza in December and January, saying five military probes found its forces "operated in accordance with international law."

But Joe Stork, a deputy director of Human Rights Watch, said: "The conclusions are an apparent attempt to mask violations of the laws of war by Israeli forces in Gaza."

The New York-based rights group said the probes "lack credibility and confirm the need for an impartial international inquiry into alleged violations by both Israel and Hamas".

It urged both sides to cooperate with a UN investigation of the conflict.

"Only an impartial inquiry will provide a measure of redress for the civilians who were killed unlawfully," Stork said.

HRW said its own investigation showed Israeli forces were responsible for "serious violations of the laws of war, including the use of heavy artillery and white phosphorus munitions in densely populated areas, the apparent targeting of people trying to convey their civilian status, and the destruction of civilian objects in excess of military need."

Some of the cases of white-phosphorus use demonstrate evidence of war crimes, the group said in a report last month.

Deputy army chief Major General Dan Harel told reporters on Wednesday the military used several hundred mortar shells and naval rounds that contain phosphorous as the active ingredient, as well about 3,000 smoke shells that contain pieces of felt dipped in phosphorous.

The military suspended the use of the phosphorous mortar shells and 76mm naval rounds on January 7 amid international controversy, Harel said, insisting those munitions had only been fired in open areas as markers and range-finders.

He insisted that the way the military used those shells as well as the smokescreen munitions whose use continued after January 7 complied fully with international law.

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

Artillery shells containing white phosphorus also struck a hospital and the headquarters of the UN Relief and Works Agency in Gaza City, the rights group claimed.

Official Palestinian figures put the Palestinian death toll at 1,475, including 943 civilians, while the Israeli military says 1,166 Palestinians were killed, including at least 709 "terror operatives."

Thirteen Israelis, three of them civilians, were killed.