JERUSALEM: Human Rights Watch said Sunday that Israel's military has fired artillery shells packed with the incendiary agent white phosphorus over populated areas of Gaza, including a crowded refugee camp, putting civilians at risk.

Researchers from the rights group said they witnessed hours of artillery bombardments on Friday and Saturday afternoon from Israel's border in which shells burst over the Jebaliya refugee camp in northern Gaza, sending out trails of burning smoke that indicates the shells contained white phosphorus.

The substance can cause serious burns if it touches the skin and can spark fires on the ground, the rights group said in a written announcement calling on Israel not to use it in crowded areas of Gaza.

Military spokeswoman Maj Avital Leibovich refused to comment directly on whether Israel was using phosphorus, but said the army was 'using its munitions in accordance with international law.'

Human Rights Watch said it had no way to investigate whether anyone was injured on the ground because its researchers have been barred by Israel from entering the territory.

Associated Press reporters in southern Gaza on Sunday saw several patients at Nasser Hospital in the town of Khan Younis with serious burns that the chief doctor said might have been caused by phosphorus. He said, however, that he did not have the resources or expertise to say with certainty what caused the injuries.

One of the patients, a young man, had severe burns that left skin on his face and body peeling off in places.

White phosphorus is not considered a chemical weapon, and militaries are permitted under laws of warfare to use it in artillery shells, bombs and rockets to create smoke screens to hide troop movements as well as bright bursts in the air to illuminate battlefields at night.

Israel is not party to a convention regulating its use. Under customary laws of war, however, Israel would be expected to take all feasible precautions to minimize the impact of white phosphorus on civilians, Human Rights Watch said.

'What we're saying is the use of white phosphorus in densely populated areas like a refugee camp is showing that the Israelis are not taking all feasible precautions,' said Marc Garlasco, a senior military analyst for the rights group. 'It's just an unnecessary risk to the civilian population, not only in the potential for wounds but also for burning homes and infrastructure.'

Garlasco was among researchers on a ridge about a mile (1.5 kilometers) from the Gaza border who observed the shelling from a 155mm artillery unit.

Some of the burning trails of smoke caused fires on the ground that appeared to go out after a few minutes, said Garlasco, who formerly worked at the Pentagon where he was in charge of recommending high-value targets for airstrikes during the 2003 invasion of Iraq.

Human Rights Watch has not been able to confirm whether there have been any civilian casualties from phosphorus. The group has a consultant working for it inside Gaza but he has been unable to move around due to the danger. Foreign journalists have also been barred from entering Gaza.

Garlasco said photos published Thursday in British newspaper The Times showed Israeli units handling American-manufactured white phosphorus shells with fuses on them.