GAZA, Feb 3 (Reuters) - The Islamist group Hamas on Wednesday formally rejected allegations it had committed war crimes during last year's fighting in Gaza, charges made in a United Nations report.

Hamas officials said the group set out in a 52-page response handed to a U.N. official in Gaza that the killing of three Israeli civilians in rocket attacks during Israel's Dec. 27, 2008-Jan. 18, 2009 offensive was an accident and military installations had been targeted.

Up to 1,387 Palestinians, including hundreds of civilians, and 13 Israelis, including three civilians, were killed in the war Israel launched with the declared aim of curbing rocket attacks on its territory.

The New York-based Human Rights Watch organisation, commenting last week after Hamas released a summary of the report, said the group's claim that its rocket attacks against Israel are not war crimes is factually and legally wrong.

Human Rights Watch, which also has criticised Israeli conduct in the Gaza war, said hundreds of rockets rained down on civilian areas in Israel where no military installations were located.

The Hamas report came in response to a U.N. mission led by South African jurist Richard Goldstone that found Israeli troops and Palestinian militants in the Hamas-ruled Gaza Strip committed war crimes.

The Goldstone report, issued in September, gave Israel and Hamas six months to mount credible investigations into their forces' conduct in the Gaza conflict or face possible war crimes prosecution in The Hague.

Israel has boycotted the U.N. investigation, calling the Goldstone report unbalanced and citing the difficulty of battling militants operating in heavily populated civilian areas.

But last Friday, Israel gave the U.N. a document detailing the Israeli military's own examinations of the Gaza offensive, disclosing it had launched 36 criminal investigations into the conduct of its forces.

The Israeli military also said that a general and a colonel had been reprimanded for authorising artillery shelling in a crowded neighbourhood in Gaza that hit a U.N. facility.

Amnesty International called the Israeli response totally inadequate.

The investigations undertaken by Israel fail to meet international standards of independence, impartiality, transparency, promptness and effectiveness, the London-based group said in a statement on its Web site. (Editing by Matthew Jones) (For blogs and links on Israeli politics and other Israeli and Palestinian news, go to http://blogs.reuters.com/axismundi)