JERUSALEM - Human Rights Watch said on Sunday that Israel has failed to properly investigate alleged crimes committed during last winter's devastating Gaza war as demanded by the United Nations.

A Palestinian woman watches during the visit of United Nations Relief and Works Agency (UNRWA) Deputy Commissioner-General Filippo Grandi (not pictured) to the central Gaza Strip February 2, 2010. REUTERS/Ibraheem Abu Mustafa (GAZA - Tags: POLITICS) "Israel claims it is conducting credible and impartial investigations, but it has so far failed to make that case," said Joe Stork, HRW's New York-based group's deputy director for the Middle East.

"An independent investigation is crucial to understand why so many civilians died and to bring justice for the victims of unlawful attacks," Stork said in a statement.

About 1,400 Palestinians were killed during the 22-day onslaught launched by Israel on December 27, 2008, aimed at halting rocket attacks from the Islamist Hamas-ruled enclave. Thirteen Israelis were killed in the fighting.

The UN General Assembly in November called on Israel and Palestinian armed groups to investigate alleged war crimes detailed in a UN report by the respected former international war crimes prosecutor Richard Goldstone.

But UN chief Ban Ki-moon said on Thursday that he had not yet determined whether both sides had adequately probed the accusations.

Last Friday, the UN chief received a 46-page report from Israel in which it denied violating international law but admitted "tragic results" due to the "complexity and scale" of fighting in heavily populated areas.

In his report, Ban highlighted Israel's assertion that two of its senior officers -- a brigadier general and a colonel -- were disciplined for the firing of white phosphorous shells toward a UN compound during the war.

Also on Friday, the UN secretary general was handed a preliminary report from the Palestinian side in which it said a commission had been set up to look into allegations that Palestinian militants committed war crimes.

Hamas also carried out an investigation into the conflict, and has previously absolved its fighters of any wrongdoing, saying they did not deliberately target civilians.

HRW has rejected Hamas's claims, saying that most of the rockets fired during the war hit civilian areas. "Civilians were the target," the group said, and "deliberately targeting civilians is a war crime."

The Goldstone report published in September also said the firing of the hard-to-aim rockets at southern Israel deliberately targeted civilians and could constitute a crime against humanity.