* Israel criticizes UN mission's mandate as one-sided
* Israel, Palestinians urged to investigate any war crimes
* Hamas says its actions were self-defense, not crimes (Adds Hamas reaction, paragraph 8)
By Louis Charbonneau
UNITED NATIONS, Sept 15 (Reuters) - Both the Israeli army and Palestinian militants committed war crimes, and possibly crimes against humanity, during the December-January Gaza war, the United Nations charged on Tuesday.
A 575-page report by a fact-finding mission organized by the Geneva-based U.N. Human Rights Council called on both sides to thoroughly investigate the allegations. Israel did not cooperate with the investigation.
"The mission concluded that actions amounting to war crimes, and possibly in some respects crimes against humanity, were committed by the Israel Defense Force," U.N. investigator Richard Goldstone told reporters.
The report also said rockets fired by Palestinian militants into Israel where there were no military targets would also constitute war crimes, and perhaps crimes against humanity.
"The rocket and mortar attacks have caused terror in the affected communities of southern Israel, causing loss of life and physical and mental injury to civilians, as well as damage to buildings and property," it said.
In response, Israel's diplomatic mission in Geneva issued a statement that criticized Goldstone's mission and explained why Israel did not cooperate with it.
"Its mandate was clearly one-sided and ignored the thousands of Hamas (Palestinian militants) missile attacks on civilians in southern Israel that made the Gaza operation necessary," the statement said.
A Hamas spokesman in the Gaza Strip rejected the suggestion that the Islamist group's firing of rockets into Israel could also amount to war crimes. He said it was self-defense, which is "a right sacred under international law."
Goldstone, a noted South African jurist, recommended that the U.N. Security Council call on Israel to fully investigate possible crimes committed by its forces. His report made clear that Palestinian authorities should do the same regarding crimes committed by Palestinian fighters.
The probes should be "independent and in conformity with international standards" and establish a committee of human rights experts to monitor any such proceedings in Israel and the Palestinian territories.
If either Israel or the Palestinians fail to do so, then the 15-nation council should refer the situation in Gaza to the prosecutor of the International Criminal Court in The Hague, the report said.
Goldstone said Israeli efforts to investigate allegations of war crimes by its soldiers had so far been "pusillanimous."
The Gaza war, codenamed Operation Cast Lead, began on Dec. 27, 2008, and ended on Jan. 18.
An Israeli human rights group, B'Tselem, said last week that 773 of the 1,387 Palestinians killed were civilians.
Israel has said 709 combatants, 295 civilians and 162 people whose status it was unable to clarify were killed. Thirteen Israelis, 10 soldiers and three civilians, died.
Israel has rejected international criticism of an offensive it said was launched to curb Hamas rocket attacks on its towns. Israel says it is investigating allegations but has not yet found cause to prosecute any of its soldiers.
Goldstone, a former South African Supreme Court and Constitutional Court judge and U.N. prosecutor, led a commission of inquiry into political violence and police hit squad activities in the early 1990s in his homeland.
The investigator said verbal attacks on his mission by Israel and pro-Israeli groups were disappointing, though understandable given the politically charged atmosphere.
Goldstone said Israel's blockade of Gaza, which began prior to the assault, amounted to "collective punishment" for the 1.5 million Gazans, most of whom rely on aid to survive. The summary said Israel's operation was directed at the people of Gaza as a whole, not just Hamas militants.
His report said Israeli forces "humiliated, dehumanized and carried out an assault on the dignity of the people in Gaza ... unlawful detentions, unacceptable conditions of detention ... obscenities and racist slogans."
It said a court could find that the blockade of Gaza is itself a crime against humanity. Israel continues to tightly restrict goods entering Gaza to staples and humanitarian supplies, saying that lifting these restrictions would allow Hamas to smuggle weapons into the Strip.
Goldstone's report also criticized Israel for firing white phosphorus incendiary shells over the U.N. compound in Gaza and for its "intentional strike" on the Al Quds hospital using high explosive artillery shells and white phosphorus.
Israeli shells hit and damaged several U.N. facilities in the Gaza Strip during the conflict.
The report said an Israeli soldier held by Hamas militants since June 2006, Gilad Shalit, has prisoner-of-war status and should be treated humanely according to the Geneva Convention. The Goldstone commission's report can be found at: here (Additional reporting by Nidal al-Mughrabi in Gaza; Editing by Alan Elsner)