By Adam Entous

JERUSALEM, Jan 18 (Reuters) - Israel tightened its closure of the Hamas-controlled Gaza Strip on Friday in response to cross-border rocket fire, preventing even U.N. humanitarian supplies from getting in, officials said.

The decision came after Israel vowed to broaden its military campaign against Gaza militants who have fired more than 110 rockets at southern Israel in the last three days.

An Israeli missile strike on Friday morning killed at least one militant in the northern Gaza Strip. In the West Bank city of Nablus, Israeli troops killed a militant linked to Palestinian President Mahmoud Abbas's Fatah movement.

The escalation in violence has prompted Abbas's Western-backed government in the occupied West Bank to warn that renewed peace talks -- spurred by last week's visit by U.S. President George W. Bush -- were in jeopardy.

The United Nations Relief and Works Agency (UNRWA), which provides food to Palestinian refugees in Gaza, said it was not allowed to deliver truckloads of humanitarian 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 had already imposed strict curbs on non-humanitarian goods entering Gaza in June when Hamas Islamists seized the coastal territory after routing Abbas's secular Fatah faction.

SIGNAL TO HAMAS

Under the Defence Ministry decision, Gaza's border crossings with Israel were closed to all goods, except for so-called "humanitarian cases" that must receive approval in advance from Defence Minister Ehud Barak, a ministry spokesman said.

"It is inconceivable that we are opening the crossings for the Palestinians and risking our people's lives," the spokesman said. "This is a signal to Hamas that it needs to contemplate if it wants to continue with this situation."

"If milk is low in Gaza, the minister will be asked to approve a milk shipment, and it will enter," the spokesman said. "The meaning is not to starve the population of Gaza."

UNRWA keep stores of supplies inside Gaza. But if the closure continues, those stocks will begin to run out, Gunness said.

Israeli Prime Minister Ehud Olmert on Thursday vowed to wage a "war" on militants.

But he gave no indication he might order a large-scale ground operation in the Gaza Strip. Olmert said Israel sought to avoid harming Palestinian civilians.

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. (Additional reporting by Nidal al-Mughrabi in Gaza and Avida Landau in Jerusalem, Writing by Adam Entous in Jerusalem; Editing by Peter Millership)

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