A UN human rights expert on Wednesday condemned a "tragic failure" by major powers to end Israel's blockade of the Gaza Strip or probe alleged war crimes committed during a military offensive one year ago.

In Geneva, the UN Special Rapporteur for the Occupied Palestinian Territories, Richard Falk, urged Israel's European and North American allies to press for the immediate end of the blockade "backed up by a credible threat of economic sanctions."

"There is no evidence of meaningful international pressure being brought to bear to end the blockade or to ensure that Israeli and Hamas officials are held accountable for alleged war crimes perpetrated during the Gaza attacks," he said in a statement.

"This represents both a tragic failure of responsibility by the powerful governments of the world and of the UN," he added.

Israel launched a devastating 22 day military offensive dubbed Operation Cast Lead on the Hamas-ruled territory on December 27, 2008.

"People of conscience everywhere, as well as governments worldwide and the United Nations, should take note of the dire situation in Gaza," said Falk.

"The ordeal of the 1.5 million residents of Gaza affected by the Israeli blockade, over half of whom are children, has been allowed to continue without any formal objection by governments and at the UN."

Falk said a three-year blockade had stopped enough food and medicine reaching civilians in Gaza, harming their physical and mental health, and was still hampering reconstruction.

He also called for full and swift implementation of the conclusions of a UN human rights investigation led by Richard Goldstone.

The Goldstone report adopted in October recommended that Israel and Hamas should face possible prosecution at the International Criminal Court, if they fail to conduct credible investigations into war crimes within six months.