* Rights group says Israel violated "laws of war" in Gaza

* Israeli army investigating use of white phosphorus

* Army knew from start civilians at risk, group says

(Adds Israeli comment)

By Adam Entous

JERUSALEM, March 25 (Reuters) - The Israeli army unlawfully fired white phosphorus shells over densely populated areas of the Gaza Strip during its recent offensive, needlessly killing and injuring civilians, Human Rights Watch said on Wednesday.

Citing Israel's use of white phosphorus as evidence of war crimes, the group said the army knew the munitions threatened the civilian population but "deliberately or recklessly" continued to use them until the final days of the Dec. 27-Jan. 18 operation "in violation of the laws of war".

The Human Rights Watch report, one of several issued by international organisations to sharply criticise Israel's conduct, called on senior military commanders to be held to account, and urged the United States, which supplied the shells, to launch its own investigation.

The Israeli army announced after the war that it would conduct an internal probe.

"We're checking the claims we received from different NGOs ... of using white phosphorus shells in illegal ways, according to those claims, and this is what we're investigating," an Israeli military spokeswoman, Major Avital Leibovich, said.

White phosphorus ignites on contact with oxygen and continues burning at up to 1,500 degrees Fahrenheit (816 degrees Celsius) until none is left or the oxygen supply is cut. It is often used to produce smoke screens, but 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 Human Rights Watch said Israel "unlawfully" fired them over populated neighbourhoods, killing and wounding civilians and damaging civilian structures, including a school, a market, a humanitarian aid warehouse and a hospital.

"In Gaza, the Israeli military didn't just use white phosphorus in open areas as a screen for its troops," said senior Human Rights Watch researcher Fred Abrahams. "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."

The group gave no precise casualty figures, citing the difficulty of determining in every case which burn injuries were caused by white phosphorous.

SHELLS LANDED IN CIVILIAN AREAS

Human Rights Watch researchers found spent shells, canister liners, and remnants white phosphorus on city streets, apartment roofs, residential courtyards and at a United Nations school.

The report documented several attacks involving white phosphorus, including one on January 4 that killed five members of Ahmad Abu Halima's family in northern Gaza, saying it found remnants of the substance at their home. "I was talking with my father when the shell landed. It hit directly on my father and cut his head off," the 22-year-old said.

The rights' group said the army knew that white phosphorus threatened civilians, citing an internal medical report about the risk of "serious injury and death when it comes into contact with the skin, is inhaled or is swallowed".

Israel launched the offensive with the declared aim of halting cross-border rocket fire by militants in the Hamas-ruled territory, home to 1.5 million Palestinians.

Over the 22 days of fighting, 1,417 Palestinians were killed, including 926 civilians, according to the Palestinian Centre for Human Rights.

Israel disputes those figures, and has accused Hamas of putting civilians at risk by using them as "human shields".

At a briefing on white phosphorus arranged by the Israeli army, retired Lt. Col. Shane Cohen, an artillery expert, said the munitions are used to create a smoke screen for troops and "wouldn't be an effective weapon".

"When we fire, we're not firing on civilians," he said.

Retired Lt. Col. David Benjamin, an international law expert, said: "Even if there is room for debate here about the legality of the use of these munitions and the circumstances of the use of these munitions, we're still far away from any talk about war crimes. And that is because war crimes deal with grave breaches of international law." (Editing by Dominic Evans) (For blogs and links on Israeli politics and other Israeli and Palestinian news, go to http:/blogs.reuters.com/axismundi)