* UN report: evidence of Israeli, Hamas war crimes

* US: Security Council need not discuss Gaza report

UNITED NATIONS, Sept 17 (Reuters) - The United States has "serious concerns" about a U.N. investigator's report accusing Israel and Palestinians of war crimes during their Gaza war, the U.S. envoy to the United Nations said on Thursday.

"The United States is reviewing very carefully what is a very lengthy document," Ambassador Susan Rice told reporters. South African jurist Richard Goldstone unveiled the report in New York this week.

"We have very serious concerns about many of the recommendations in the report," Rice said.

The Goldstone commission said both the Israeli army and Palestinian militants had committed war crimes, and possibly crimes against humanity, during the December-January war in the Gaza Strip. It said both had terrorized and killed civilians.

It urged the U.N. Security Council to refer the allegations to the International Criminal Court in The Hague if either Israel or Palestinian authorities failed to investigate and prosecute those suspected of such crimes within six months.

Israel had criticized the investigation from the start and refused to cooperate with a mission whose mandate it said was "clearly one-sided." Both Israel and the Palestinian militant group Hamas rejected the 575-page document.

Goldstone's mission was organized by the Geneva-based U.N. Human Rights Council, a body Israel and the United States have long criticized for what they say is its anti-Israeli bias.

Earlier this year, the United States successfully ran for a seat on the council, vowing to try to change the U.N. rights body from within. Under President Barack Obama's predecessor, George W. Bush, the United States had boycotted the panel.

'UNACCEPTABLE'

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.

Rice said the Human Rights Council's approach to the Gaza war investigation was deeply flawed.

"We have long expressed our very serious concern with the mandate that was given by the Human Rights Council prior to our joining the council, which we viewed as unbalanced, one-side and basically unacceptable," she said.

She also rejected one of the key recommendations in Goldstone's report -- that the Security Council should now take up the matter. Rice made clear that Washington saw no point in the 15-nation council even discussing the report.

"The appropriate venue for this report to be considered is the Human Rights Council," she said.

Western diplomats said temporary council member Libya was interested in debating the Goldstone report. As a result, they said, the council might have to engage in a symbolic discussion of it but would almost certainly take no action.

Rice said the focus should be the future.

"This is a time to work to cement progress toward the resumption of (Israeli-Palestinian peace) negotiations and their early and successful conclusion," she said.

Obama might meet with Israeli Prime Minister Benjamin Netanyahu and Palestinian President Mahmoud Abbas on the sidelines of the U.N. General Assembly next week to restart stalled peace talks. (Editing by Alan Elsner)