The United Nations humanitarian chief warned Thursday of an impending humanitarian disaster if Egypt succeeds in blocking the tunnels that pass under it's border into the Gaza Strip.

John Holmes said that as bad as the hundreds of tunnels that bypass the Israeli blockade are, Gaza would have difficulty surviving if Egypt succeeds in blocking them because they are a conduit for badly-needed food, medicine and commercial goods.

He repeated calls for Israel to end its blockade of the Palestinian territory.

"If those tunnels were blocked, however undesirable they may be, and however undesirable the effect they're having on the Gazan society and Gazan economy, the situation without the tunnels would be completely unsustainable," said Holmes, who visited the region for four days earlier this month.

The tunnels are also widely believed to be used for smuggling cash and weapons to Hamas, which wrested control of Gaza from the Palestinian Authority in 2005 and refuses to recognize Israel. The Israeli government has repeatedly tried to shut the tunnels down.

Egypt has a fence along Gaza's southern border and is reinforcing the area with underground metal plates to try to block the tunnels. Egypt's President Hosni Mubarak has defended the move as necessary for his nation's security following a series of terrorist attacks on nearby tourist resorts.

Holmes said it was very frustrating to see that there has been almost no rebuilding in Gaza, as a result of the Israeli blockade, since the three-week conflict that ended in Jan. 2009, leaving 13 Israelis and almost 1,400 Palestinians dead.

"What people in Gaza want to see is the opening of the crossings ... not only for goods but for people because they are living ... in a large open-air prison," Holmes said.

Holmes said the blockade is not helping Israel's security or weakening Hamas' hold on the Palestinian territory.

"The blockade of Gaza does not serve any purpose in my view in security terms for Israel because anything can come in through those tunnels at the moment, nor is it weakening the hold of Hamas on Gaza," he said. "That's perfectly clear when you're there."

Israel has linked the release of Israeli soldier Gilad Shalit, who was captured by Palestinian militants in 2006 and is being held by Hamas, to the blockade, saying it will not consider easing the embargo until Shalit is home. Holmes said the United Nations has called for the release of Shalit and urged that he be treated in accordance with the Geneva conventions.

"But the link between that and the fate of the 1.5 million Palestinians in Gaza does not seem to us a reasonable one," he said