By Anthony Deutsch and Dan Williams

THE HAGUE/JERUSALEM (Reuters) - The possibility of a war crimes investigation into the conduct of Israeli forces in Gaza, until recently unthinkable, has grown after the Palestinians said this week they wanted to become a party to the International Criminal Court.

The world's permanent war crimes court in The Hague declined two years ago to investigate allegations against the Israeli military in 2008-2009, citing the uncertain legal status of the Palestinian Authority.

A lot has changed since then.

Fresh allegations of war crimes have flowed in recent weeks from fighting in Gaza, where Israel responded to a surge in rocket attacks by Hamas militants with air strikes and a ground incursion. The Palestinians this week unexpectedly said they are just one procedural step away from ICC membership.

The legal groundwork for such a move was laid in Nov 2012 when the 193-member U.N. General Assembly overwhelmingly approved the de facto recognition of the sovereign state of Palestine by upgrading the Palestinian Authority's observer status to "non-member state" from "entity.”

If the Palestinians were to sign the ICC's founding treaty, the Rome Statute, the court would have jurisdiction over crimes committed in the Palestinian territories.

With Palestinian authorization, an ICC investigation could then examine events as far back as July 1, 2002, when the court opened with a mandate to try individuals for war crimes, crimes against humanity and genocide.

"If Palestine applies it will be admitted to the ICC," John Dugard, international law professor and a former U.N. Special Rapporteur for the Palestinian territories, told Reuters.

"The U.N. has spoken and it has recognized the state of Palestine and it is now for the ICC to admit Palestine. I cannot see how that can be resisted."

Dugard said the Palestinians could then ask prosecutors to investigate alleged crimes in July and August in Gaza, but also the legality of Israeli West Bank settlements.

Story continues

"The settlements are an ongoing crime and it is quite clear that the settlements constitute a war crime under the Rome Statute and that is what Israel is desperately worried about," Dugan said.

Israel says the settlements are legal, as it captured the West Bank from Jordan, rather than a sovereign Palestine, in the 1967 Middle East war.

ISRAELI DEFENSE

War crimes accusations accompanied Israel's incursion into Gaza in July. Given the far higher toll of civilian deaths and destruction on the Palestinian side, a U.N. inquiry was launched into human rights violations.

Israeli officials said force was used proportionately. But Israel's closest ally, Washington, called the shelling of a U.N.-run school "disgraceful".

State Department spokeswoman Jen Psaki called for a probe into attacks on U.N. schools in densely populated Gaza. "While they have the right to defend themselves, there is more they can do" to prevent civilian deaths, she said.

Gaza officials say 1,874 Palestinians, most of them civilians, have been killed, while Israel counted 64 soldiers and three civilian deaths on its side.

Israeli officials said the hostilities were justified self defense against militant attacks and that all efforts have been made to avoid civilian deaths. Roughly 47 percent of those killed in Gaza were combatants, Israel said.

One Israeli official, who spoke on condition of anonymity because the legal strategy is confidential, said the Israeli government is planning a defense of the Gaza operation and that counter-claims, including against the administration of President Mahmoud Abbas, could follow if the ICC launches a case.

"We are talking about terrorism involving officials, security personnel and others, from his administration, and emanating from areas under his control,” the official said.

Israel has taken great care to adhere to the laws of war and conducts internal investigations to ensure military personnel follow rules of conduct and morality, the official said.

Israeli forces destroyed or damaged some 3,000 homes in Gaza during this war, according to Palestinian estimates. The Israelis don’t dispute attacking homes, but Israeli jurists said they were pre-vetted and approved as legitimate military targets.

Richard Kemp, a retired British army colonel who formerly commanded forces in Afghanistan and Iraq and has been studying the Israeli military's doctrine, believes the operations were conducted within the law.

"That does not mean that in the heat of the moment it would not be possible for an individual soldier or commander to act outside the laws of war, " he said.

"That kind of adherence to the law does not preclude accidental killing of civilians. If it is done accidentally, if it is not a general theme or policy of negligence, then that is not a deliberate breach. Accidents happen in every army.”

PALESTINIANS TOO COULD FACE PROSECUTION

The ICC, with 122 member countries, is a court of last resort, meaning that it intervenes only when a state is unable or unwilling to prosecute alleged crimes. The United States, China and Russia have notably not joined.

On top of criticism of being sluggish, prosecutors also have faced criticism from analysts for only bringing charges against Africans, while atrocities in conflicts in the politically-sensitive Middle East, such as Iraq, Syria and Egypt, go unpunished.

The ICC defends its procedures.

"The selection of situations and cases, and persons to be investigated, is always an independent prosecutorial decision based on the Rome Statute legal framework and the evidence collected. Geographical and political consideration will thus never form part of any decision making," the prosecutor’s office said in a statement to Reuters.

ICC membership has been described by diplomats and officials as the Palestinian “nuclear option” because it is the key leverage the Palestinians hold in negotiations. It would also expose the Palestinians themselves to possible prosecution.

Nearly a month of fighting in Gaza "left us no choice" but to seek a case against Israel at the ICC, Palestinian Foreign Minister Raid al-Malki said on Tuesday after meeting with prosecutors to discuss joining the court.

"An investigation by the ICC is becoming crucial in the absence of a real system of accountability, due to the existence of a pervasive culture of impunity given to Israel and resulting from the lack of action by the international community," he said.

Malki said "there is no difficulty for us to show or build the case. Israel is in clear violation of international law."

Factions within the Palestinian Authority are divided about joining the ICC and analysts say Hamas is unlikely to agree if its leaders might be prosecuted.

Hassan Al-Aouri, legal advisor to President Mahmoud Abbas, told Voice of Palestine radio on Wednesday that Hamas and the Islamic Jihad have rejected the idea of ICC membership.

"We are trying to convince them it is necessary to go to the court," Aouri said. "At the end of the day the president is the one in charge and the leadership will study the issue and will make a decision."

Even if the ICC were to issue arrest warrants against officials in Israel and the Palestinian territories, it is uncertain how they would be brought to trial. The ICC has no police force and relies on the cooperation of member states to transfer suspects to The Hague.

(Additional reporting By Nidal Al Mughrabi in Gaza, Michelle Nichols in New York, Jussi Rosendahl in The Hague and Ali Sawafta in Ramallah.)