In early January, a week into Israel’s war in Gaza, the home of Sabah Abu Halima was hit by an Israeli shell. Ms. Abu Halima, the matriarch of a farming family in the northern Gaza area of Beit Lahiya, was caught in an inferno that burned her husband and four of their nine children to death.

But as she lay in a bed on the third floor of an annex to Shifa Hospital in Gaza City on Wednesday, bandaged all over and in terrible pain, it was less the magnitude of her loss than the source of the fire that was drawing attention, not only from her doctors but also from human rights organizations and even the Israeli military.

Though there has been no independent confirmation, Palestinian officials say her family was hit by white phosphorus, a weapon that militaries use widely to obscure the battlefield but that is also limited under an international convention that bans targeting civilians with it.

The Israeli military issued a short statement on Wednesday, saying it was investigating whether its use of phosphorous weapons was improper and reiterating that it was “obligated to international law” in the matter. Early in the war, Israeli officials would not confirm whether the military was using white phosphorus at all, but said only that it was using weapons in legal ways.

Meanwhile, Amnesty International said it found “indisputable evidence of widespread use of white phosphorus in densely populated residential areas in Gaza City and in the north.” In a statement, it said its investigators “saw streets and alleyways littered with evidence of the use of white phosphorus, including still-burning wedges and the remnants of the shells and canisters fired by the Israeli Army.” It called such use a likely war crime and demanded a full international investigation.

The use of white phosphorus and other incendiary weapons is covered in one protocol of a 1980 international treaty, the Convention on Certain Conventional Weapons, that bans making civilians “the object of attack” by such arms. More broadly, though, international officials have acknowledged that militaries can legitimately use the substance in some cases.

Phosphorus rounds are usually used to spread a thick, white smoke to screen military actions and mark specific areas. Military experts say phosphorus is often particularly useful in urban warfare, in part because it creates tall columns of smoke that can obscure upper-story windows.

But human rights groups harshly criticize its use, saying that the horrible burns and the widespread fires that phosphorus causes make it a menace to civilians. Peter Herby, the head of the Arms Unit for the International Committee of the Red Cross, said in a statement that his agency would not comment publicly on whether it considered Israel’s use of white phosphorus a violation of international humanitarian standards, pending further investigation.

In Gaza, Ms. Abu Halima said that when her family was hit, “fire came from the bodies of my husband and my children.”

“The children were screaming, ‘Fire! Fire!’ and there was smoke everywhere and a horrible, suffocating smell,” she said. “My 14-year-old cried out, ‘I’m going to die. I want to pray.’ I saw my daughter-in-law melt away.”

Dr. Nafez Abu Shaban, head of Shifa’s burn unit, said the family’s burns, which he and an assisting doctor from Egypt had treated, were of a kind he had never encountered, reaching to the muscle and bone.

“They were deeper and wider than anything I had seen; a bad odor came from the wounds and smoke continued to come out of them for many hours,” he said in his office around the corner from Ms. Abu Halima’s sickbed.

He added, “We took out a piece of foreign matter that a colleague identified as white phosphorous.”

Dr. Shaban said that dozens of such cases came to Shifa during the war and that his unit was unprepared to handle them. Many of the burn patients have been sent to Egypt and abroad from there. In a few cases, he said, seemingly limited burns led to the patients’ deaths.

The doctors discovered that the best way to deal with such burns was to get the patients immediately into surgery and clean the areas well. Initial attempts to dress phosphorous burns like normal ones made them worse.

Part of what makes white phosphorus controversial is that it can be difficult to control how wide the effects are. When the shells explode in the air, they disperse pieces of felt soaked in phosphorus — larger version of the shells contain more than 100 of them — that can land on people and cause intense burning, according to Chris Cobb-Smith, a British Army veteran who is here as part of Amnesty International’s investigative team.

The newspaper Haaretz reported Wednesday that one focus of the Israeli military’s inquiry was the use of white phosphorus by a reserve brigade that fired about 20 such shells in Beit Lahiya, where Ms. Abu Halima lives. Col. Shai Alkalai, an artillery officer, is leading the investigation.

Haaretz said about 200 such shells were fired in the fighting, nearly all at orchards where Hamas gunmen and rocket-launching crews were taking cover.

The article added that some of the rounds used were recently acquired 120-millimeter phosphorus shells that have a computerized targeting system attached to a G.P.S. unit. It quoted commanders as saying the shells had been effective but were apparently also responsible for the strike on a United Nations school that killed two and a friendly-fire episode that seriously wounded two Israeli officers.

Donatella Rovera, Amnesty International’s researcher for Israel, the West Bank and Gaza, said in an interview, “We don’t know why they used them, but we do know that it could constitute a war crime.”

She added, “It is not a banned weapon, but it matters how you use it and there is no reason to use it in such densely populated areas. We want a full impartial investigation, not one by the army that used it.”

Ms. Abu Halima said that on Tuesday some relatives went to her home and found it destroyed. They then properly buried the dead.

She wept with fury, saying that as farmers she and her family had good relations with Israelis, selling them produce in past years. But now, she said, she wants to see Israel’s leaders — she named the foreign minister and president — “burn like my children burned.”

“They should feel the pain we felt.”