A government report released Thursday insisted that "incessant" Hamas rocket attacks forced Israel to hit Gaza hard earlier this year, countering charges of war crimes but acknowledging that more than a dozen criminal inquiries are underway. The 160-page report was called the first comprehensive Israeli government study of Operation Cast Lead in December and January that killed more than 1,100 Palestinians. It was an attempt to answer charges from Palestinians, the UN and human rights groups that Israeli forces committed war crimes and violated international law during the three-week operation. Charges have included indiscriminate and intentional firing that killed civilians and destroyed property. During the conflict, Israeli warplanes, tanks and artillery obliterated Palestinian government buildings and destroyed or damaged thousands of apartments, houses, businesses and factories. Israeli officials have acknowledged that their soldiers used additional firepower to keep their own casualties down. Ten Israeli soldiers were killed during the conflict, along with three civilians who died in rocket attacks. The scope of the destruction triggered a flood of scathing reports from human rights groups. Defending Israeli actions, the government report said it was not meant to be an "assertion of infallibility," but rejected the charges one by one, attributing excessive damage and casualties to understandable wartime mistakes. The report said Israel is investigating about 100 complaints and has opened 13 criminal inquiries. A military statement Thursday said criminal cases under investigation now number 15. "Israel had both a right and an obligation to take military action against Hamas in Gaza to stop Hamas' almost incessant rocket and mortar attacks," the report's executive summary stated, noting that 12,000 rockets and mortars were fired at Israel between 2000 and 2008, including nearly 3,000 in 2008 alone. "Under international law, Israel had every right to use military force to defend its civilians," the report said. The report said 1 million Israelis were threatened by Hamas rockets, tens of thousands were traumatized and thousands fled their homes. It called the offensive, which began December 26 and lasted three weeks, a "necessary and proportionate" response. Hamas official Mushair al-Masri rejected the report, repeating the charge that Israel committed war crimes in Gaza. "This report is ridiculous and stupid and does not deserve a response," he said Thursday. Palestinian Authority President Mahmoud Abbas has denounced the Hamas rocket fire but charged that Israel's response was excessive. The report analyzes at length the steps Israeli forces took to minimize civilian casualties in Gaza, while claiming that some such casualties were inevitable because Hamas fighters took up positions in crowded neighborhoods. It said international law is violated only "when there is an intention to target civilians," and Israel had no such intention - in contrast to Hamas targeting Israeli civilians with its rockets. The report explains damage to UN facilities by blaming Hamas for setting up rocket launchers nearby. In one of the specific case studies, the Israeli report dismissed charges that dozens of Palestinian civilians were killed or wounded by white phosphorous shells, which are used to lay down smoke screens. In a footnote, the report claimed, "There appear to have been no documented deaths in Gaza resulting from exposure to white phosphorus itself." The report acknowledged that shell casings with phosphorus residue could have hurt some people and started fires, but "it does not appear that damage from this use can be regarded as excessive." Amnesty International is among the groups charging Israel with war crimes. In a report this month, the group deplored Israel's use of less-precise artillery shells and white phosphorous in built-up areas. It also accused Israeli forces of using Palestinians as "human shields" and frequently blocking civilians from receiving medical care and humanitarian aid. Internal investigations into the use of white phosphorous have "uncovered no violations of international law," the report said, but noted that some inquiries are still in progress. The report detailed steps aimed at limiting civilian casualties, counting 2.5 million leaflets and 165,000 phone calls to civilians warning them to leave targeted areas. Also, it said, humanitarian aid flowed into Gaza throughout the conflict. In related news, the IDF has decided that in the event of a future conflict it will issue more detailed warnings to Palestinians before air strikes in residential areas. In recent discussions on the results of the operation, senior IDF officers have called to make improvements to procedures and to provide more details in the flyers to ensure that the Palestinians realize that their lives are at risk. Some of the flyers may henceforth include details on routes that the Palestinians can use to flee an area which is scheduled to be invaded. Yaakov Katz contributed to this report.