The IDF rejected accusations on Sunday that it is using illegal arms in its war against Hamas in the Gaza Strip. Human Rights Watch earlier accused the army of firing artillery shells packed with white phosphorus, an incendiary agent, over populated areas, including a crowded refugee camp, and putting civilians at risk. Researchers from the NGO said they witnessed hours of artillery bombardments on Friday and Saturday afternoon from just within Israel in which shells burst over the Jabalya refugee camp in northern Gaza, sending out trails of burning smoke that indicated 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. In response, the IDF released a statement saying that it only used "munitions in accordance with international law." The IDF came under similar criticism during the Second Lebanon War for its use of cluster bombs. White phosphorus is not considered a chemical weapon, and militaries are permitted under the 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. 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 Gaza. Associated Press reporters in southern Gaza saw several patients at Nasser Hospital in Khan Yunis on Sunday 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. Israel is not party to a convention regulating the use of white phosphorus. Under customary laws of war, however, Israel would be expected to take all feasible precautions to minimize its impact 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 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."