Israel has blocked a United Nations fact-finding mission to the Gaza Strip led by Desmond Tutu, UN officials said Monday.

But Israel said no final decision has been made.

Tutu and a six-person team were supposed begin on the weekend to investigate the killing of 19 civilians in the northern Gaza town of Beit Hanoun in November.

But Israel has refused to grant Tutu the necessary travel clearance, said officials in two UN departments, speaking on condition of anonymity.

Tutu, a Nobel Peace Prize winner and anti-apartheid campaigner, was supposed to report his findings to the UN Human Rights Council by Friday.

It is unclear whether Israel will allow the fact-finding mission to take place at a later date.

"Israel heard that they decided not to come," Israeli Foreign Ministry spokesman Mark Regev said. "We had not given them a negative response, our final decision was pending."

Israeli officials have expressed concern that Tutu's mission was only entrusted with investigating alleged human rights violations committed by Israel and not also by Palestinian militants.

"We had a problem not with the personalities, we had a problem with the institution," Regev said. "We saw a situation whereby the human rights mechanism of the UN was being cynically exploited to advance an anti-Israel agenda.

"This would do the Israelis, the Palestinians and peace in the Middle East no good at all. This would also have done nothing to serve the interest of human rights."

The UN authorized the mission last month, asking Tutu to assess the situation of victims, address the needs of survivors and make recommendations on ways to protect Palestinian civilians against further Israeli attacks.

Israeli tanks opened fire in a residential neighbourhood in Beit Hanoun, killing Palestinians children and adults as they slept. Israel has said the killings were unintentional.

The shelling came as Israeli troops wound up a weeklong incursion meant to curb Palestinian rocket attacks on Israel. The Israeli army claimed Beit Hanoun was a rocket-launching stronghold.

Tutu, a retired Anglican archbishop, was a vocal anti-apartheid campaigner in South Africa. When apartheid ended, he chaired the country's Truth and Reconciliation Commission. He won the Nobel Peace Prize in 1984.