Israel fired white phosphorous shells indiscriminately over densely populated areas of Gaza in what amounts to a war crime, Human Rights Watch said in a report Wednesday.

The New York-based group called on the United Nations to launch an investigation into alleged violations of the rules of war, both by Israel and Hamas, during the three-week Gaza war.

The Israeli military said Wednesday that the shells were used in line with international law.

"The claim that smoke shells were used indiscriminately, or to threaten the civilian population, is baseless," the military said in a statement.

International law permits the use of phosphorous weapons as flares or to create smoke screens masking the movement of troops.

However, Human Rights Watch said Israeli troops frequently fired the shells over densely populated areas. The firing "was indiscriminate and is evidence of war crimes," the report said.

The group documented only some of the cases, including white phosphorous shells fired at a Gaza City hospital, the U.N. headquarters, a school and a market. In six attacks, 12 civilians were killed and dozens wounded, said Human Rights Watch researcher Fred Abrahams.

Each shell bursts into 116 burning white phosphorous wedges, over a radius of more than 135 yards. The wedges burn on contact with oxygen, creating intense heat, and can cause severe burns. The phosphorous kept burning for many days, and was still smoldering well after Israel's withdrawal on Jan. 18.

Abrahams said the Israeli military was aware of the destructive nature of white phosphorous. Army medical officers warned during the war that the weapon is "potentially extremely destructive to tissue," according to an internal army document attached to the report.

Israel's use of white phosphorous violated the laws of war, Abrahams said.

"They knew perfectly well what danger white phosphorous poses to civilians," Abrahams said. "Their own documents prove it. They know that these areas were densely populated. Yet they fired it not once, not twice, but repeatedly into densely populated areas."

Israel could have used less dangerous smoke screens produced by an Israeli company, Abrahams said.

He called for an investigation by the U.N. secretary-general or the U.N. Security Council into all of Israel's alleged violations of the rules of war, including the use of white phosphorous.

The United States supplied the white phosphorous to Israel and should launch its own investigation into whether the shells were used illegally, he said.

Hamas should also be investigated for war crimes, including indiscriminate rocket fire into Israeli border towns, Human Rights Watch said.

Lt. Col. Shane Cohen, a reserve artillery officer, noted that white phosphorous shells are also used in the British and U.S. armies. He said shells would not be fired as smoke screens if civilians are present.

"The enemy might be behind the civilians, and you want to put the smoke screen in the middle, so you're going to think more than twice, and nearly for sure, you're not going to be firing it," he said.

Israel launched its Gaza offensive on Dec. 27 in an attempt to halt rocket fire and weaken the territory's Hamas rulers. More than 1,400 Palestinians, including more than 900 civilians, were killed in the war, according to a Palestinian human rights group. Thirteen Israelis were also killed.