Israel tightened its closure of the Hamas-controlled Gaza Strip today in response to cross-border rocket fire.
Israel has vowed to broaden its campaign against Gaza militants who have fired more than 110 rockets at southern Israel in the past three days.
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," a UNRWA spokesman said.
Israel had already imposed strict curbs on non-humanitarian goods entering Gaza in June when Hamas Islamists seized the coastal territory after routing Palestinian President Mahmoud Abbas's secular Fatah faction.
"It is inconceivable that we are opening the crossings for the Palestinians and risking our people's lives," an Israeli defence ministry spokesman said. "This is a signal to Hamas that it needs to contemplate if it wants to continue with this situation."
An Israeli missile strike today 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 Fatah.
The escalation in violence has prompted Mr Abbas's Western-backed government in the occupied West Bank to warn that renewed peace talks - spurred by last week's visit by President George W. Bush - were in jeopardy.