JERUSALEM - Israel tightened its closure of the Hamas-controlled Gaza Strip on Friday in response to cross-border rocket fire by militants, preventing even U.N. humanitarian supplies from getting in, officials said.

Israel's stepped up campaign in the coastal territory has prompted the Palestinians to warn that peace talks between President Mahmoud Abbas and Prime Minister Ehud Olmert, spurred by a visit by U.S. President George W. Bush, were in jeopardy.

Israel has killed more than 30 Palestinians in Gaza since Monday, including one militant and a civilian in a missile strike on Friday morning.

The Israeli army says it is targeting Gaza militants who have fired more than 110 rockets into the Jewish state in the last three days alone. In the West Bank city of Nablus, Israeli troops killed a militant linked to Abbas's Fatah movement.

The Israeli Defence Ministry ordered all of the border crossings with Gaza closed and said only "humanitarian cases" which receive Defence Minister Ehud Barak's personal approval will be allowed through.

"If milk is low in Gaza, the minister will be asked to approve a milk shipment, and it will enter," a Defence Ministry spokesman said.

Gaza is home to 1.5 million people, most of whom depend on foreign aid.

The United Nations Relief and Works Agency (UNRWA), which provides food to refugees, said it was not allowed to deliver truckloads of supplies on Friday morning as it usually does.

"Gaza is completely shut down. This will only add to an already dire situation," said UNRWA spokesman Christopher Gunness.

Israel has imposed strict curbs on non-humanitarian supplies to Gaza since June when Hamas Islamists seized the coastal territory after routing Abbas's forces.

Many essentials have been getting in, either with Israeli approval or through smuggling, though supplies are limited and prices have risen sharply.

Before Friday's announcement, UNRWA was allowed to bring an average of 15 truckloads of supplies into Gaza per day. Gunness said it was unclear when the flow of aid would resume.

'SIGNAL TO HAMAS'

"It is inconceivable that we are opening the crossings for the Palestinians and risking our people's lives," the Defence Ministry spokesman said.

"This is a signal to Hamas that it needs to contemplate if it wants to continue with this situation... The meaning is not to starve the population of Gaza," the spokesman added.

Gunness said UNRWA has enough supplies in Gaza to last up to two months, but that the U.N. rations provide only a portion of the food that recipients need.

Olmert vowed on Thursday to wage a "war" on Gaza militants. But he gave no indication he might order a large-scale ground operation.

One of the makeshift rockets fired from Gaza landed 40 meters (44 yards) from an Israeli nursery school on Friday. Government spokesman David Baker issued a statement saying Israel "will not be deterred from taking any necessary steps" to halt the fire from Gaza.

An Ecuadorean volunteer on an Israeli farming community bordering Gaza was shot dead by a Palestinian sniper on Tuesday.

Abbas's government has condemned the Israeli operations as "a slap in the face" to efforts by Bush to achieve a peace treaty by year's end.