Israel has failed to show it will conduct an impartial investigation of allegations that it committed war crimes during Operation Cast Lead last winter, an international human rights group said Sunday.

The New York-based Human Rights Watch said inquiries by Israel's military have largely focused on possible wrongdoing by individual soldiers without looking into high-level decisions that led to large numbers of civilian casualties, such as artillery fire into populated areas.

Israeli investigators missed an important piece of evidence in one of the most contested incidents of the war, in which Gaza's only flour mill was severely damaged by Israeli fire, said Human Rights Watch, which discussed the ongoing investigations with Israeli military lawyers last week.

"Israel claims it is conducting credible and impartial investigations, but it has so far failed to make that case," said Joe Stork, deputy Middle East director for Human Rights Watch. "An independent investigation is crucial to understand why so many civilians died and to bring justice for the victims of unlawful attacks."

Foreign ministry spokesman Yigal Palmor dismissed the group's findings, saying that the military is investigating "in full transparency everything that needs to be investigated."

Israeli human rights groups have also called for an independent probe. Prime Minister Binyamin Netanyahu has so far given no indication that he plans to authorize such an investigation.

A team of UN investigators, headed by veteran war crimes prosecutor Richard Goldstone, said it found evidence that both sides violated the laws of war. The team said Israel used disproportionate force and deliberately targeted civilians, while Hamas indiscriminately fired rockets at Israeli civilians.

Both Israel and Hamas denied the allegations, but were told by the UN General Assembly in November to launch credible investigations or face possible Security Council action, such as a referral to the International Criminal Court.

Both submitted reports about their efforts last week, but UN Secretary General Ban Ki-moon said he could not determine whether the investigations were credible.

Human Rights Watch said it was still reviewing the Hamas response, but rejected Hamas' assertion that it didn't intend to harm Israeli civilians. Hamas fired hundreds of rockets during last winter's war, killing three Israeli civilians.

Israel has said it has conducted more than 140 inquiries into alleged violations of international law during Cast Lead, including 36 criminal investigations. One resulted in a conviction, a relatively minor case of a soldier stealing a credit card and charging $400 on it. Twenty-nine cases remain open, the military has said.

Two high-ranking officers were reprimanded for approving the firing of artillery shells toward a UN compound.

Stork, the Human Rights Watch official, said an independent investigation should look into the high-level decisions that led to civilian casualties, such as firing heavy artillery and white phosphorus shells in populated areas.

"For those decisions and policies, senior military and political decision-makers should be held responsible," he said.

The Goldstone report alleged that Israel bombed Gaza's only flour mill from the air, as part of a deliberate attempt to damage the civilian infrastructure in Gaza. Israel said the mill was struck inadvertently by a tank shell during fighting with Hamas terrorists.

Human Rights Watch said UN mine defusal experts visited the mill two days after the strike and found the front half of a 500-pound aircraft bomb on the upper floor. Human Rights Watch also released a video, taken by the mill's owner, and said it appears to show the remains of an aerial bomb.

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