Israel is preventing UN investigators from speaking to witnesses and victims of the October 7 Hamas attack, former UN rights chief Navi Pillay, who is chairing a three-person probe, said Tuesday.

The unprecedented open-ended Commission of Inquiry was established by the UN Human Rights Council in May 2021 to investigate all alleged violations of international humanitarian and human rights law in Israel and the Palestinian territories.

"I deplore the fact that people inside Israel who wish to speak to us are being denied that opportunity, because we cannot get access into Israel," Pillay said.

The investigation was briefing diplomats at the UN in Geneva on its progress and said that since October 7, it had focused entirely on the current Gaza conflict.

"So far as the government of Israel is concerned, we have faced not merely a lack of cooperation but active obstruction of our efforts to receive evidence from Israeli witnesses and victims to the events that occurred in southern Israel," said Chris Sidoti, one of the three members of the inquiry.

The bloodiest-ever Gaza war began with Hamas's unprecedented October 7 attack against Israel which resulted in the deaths of 1,170 people, mostly civilians, according to Israeli figures.

Israel's retaliatory offensive has killed at least 33,797 people in the Palestinian territory, mostly women and children, according to the health ministry in Hamas-run Gaza.

Pillay, a South African former High Court judge, said the commission was investigating alleged crimes during the Hamas attack as well as those allegedly committed by Israeli forces in the Gaza Strip and in the West Bank.

Pillay also served as an International Criminal Court (ICC) judge and presided over the International Criminal Tribunal for Rwanda.

Sidoti, speaking via video-link, said the investigation had found it difficult to collect evidence from large numbers of witnesses.

"I use this opportunity to appeal again both to the government of Israel to cooperate, and to victims and witnesses to the events in southern Israel to contact the Commission of Inquiry so that we can hear what they have experienced," he said.

Sidoti also said the investigators began collecting digital evidence early on October 7, some of which has since "disappeared from the internet".

"If it had not been collected on that day, it would not have been able to be collected," the former human rights commissioner of Australia said.

Pillay said the commission had shared more than 5,000 documents with the ICC in The Hague, collected between October and December 2023.

The commission is due to present its first set of findings to the Human Rights Council in June.