LONDON (AFP) - A group of the world's most experienced judges and human rights investigators called Monday for a full international investigation into alleged abuses of international law in the Gaza conflict. In an open letter supported by Amnesty International, they stress the need for an investigation into "all serious violations of international humanitarian law committed by all parties to the conflict." The letter argues that the UN investigation "should not be limited only to attacks on UN facilities." The signatories - who have led investigations of crimes committed in former Yugoslavia, Kosovo, Darfur, Rwanda, Sierra Leone, South Africa, East Timor, Lebanon and Peru - say they were "shocked to the core" by the events in Gaza . They argue they "have seen at first hand the importance of investigating the truth and delivering justice for the victims of conflict and believe it is a precondition to move forward and achieve peace in the Middle East." And they call on world leaders "to send an unfaltering signal that the targeting of civilians during conflict is unacceptable by any party on any count." The signatories include Richard Goldstone, the former chief prosecutor of the International Criminal Tribunals for the former Yugoslavia and for Rwanda, and Antonio Cassese, the first president of the International Criminal Tribunal for the former Yugoslavia and head of the UN Inquiry on Darfur. The Israeli military launched a massive 22-day offensive on Gaza in late December in response to the firing of rockets into Israel by Hamas activists. Meanwhile, Israel's outgoing PM Ehud Olmert said on Sunday that his govt had been ready to sign a peace deal with the Palestinians but that they had lacked the courage to do so. "The peace negotiations with the Palestinians had made a big step forward and were more interesting and deeper than any talks held in the past by any government, including (then premier, now outgoing Defence Minister) Ehud Barak at Camp David (in the United States in 2000)," Olmert said. "The fact that we haven't reached one (a peace agreement) so far is due to the weakness and lack of courage on the part of the Palestinian leaders," he told the weekly cabinet meeting, likely to be one of his last. "We were ready to sign a peace deal but the Palestinians unfortunately did not have the courage to do so," he said. There was no immediate reaction from the Palestinians, although Western-backed president Mahmud Abbas charged in November that he had been unable to reach agreement with Israel on a single issue in 12 months of negotiations. Olmert reiterated that Israel "will have to make dramatic and painful concessions to reach a peace agreement."