Israeli jets and navy vessels kept up the attacks on targets in Gaza yesterday morning as the death toll reached 310 with more than 900 wounded.

But Hamas officials threatened to retaliate by assassinating senior Israeli officials. A Hamas spokesman, Fatah Hamad, singled out the Foreign Minister, Tzipi Livni, and the Defence Minister, Ehud Barak.

An Israeli Arab man was killed and seven people wounded after a rocket launched by militants in Gaza hit the Israeli port city of Ashekelon to the north.

And a Palestinian man was shot dead yesterday after he infiltrated an Israeli settlement near Ramallah in the West Bank and stabbed three settlers, one of whom was wounded seriously.

The Israel Defence Force chief of staff, General Gabi Ashkenazi, continued to hold out the possibility of a ground operation, with thousands of troops assembled outside the Gaza Strip.

The Israeli cabinet has approved the call-up of about 7000 reserves in the event of a ground operation.

Israeli military officials said that since the beginning of Operation Cast Lead on Saturday, more than 250 targets had been destroyed.

Early yesterday Israeli jets attacked approximately 40 Hamas targets, including two buildings at the Islamic University.

The military said these buildings were used for manufacturing rockets and explosives, and high-ranking Hamas had officers held meetings there.

Mr Barak pledged that Israel would continue fighting until the goals determined for the operation have been accomplished.

Captain Benjamin Rutland, a senior defence force spokesman, told the Herald that the purpose of the assault was to

minimise the capacity of Hamas and other militant groups in Gaza to fire rockets on Israel and to "create an improved security situation over the long term".

"The targets which have been attacked have been targets associated with Hamas including command posts, training facilities, weapons manufacturing facilities, and weapons storage facilities," Captain Rutland said.

"The IDF does not target non-combatants. The majority of the casualties have been uniform wearing members of the Hamas terror organisation responsible for attacks on Israel," he said.

But the United Nations office in Gaza said the official number of civilian casualties had risen to 51. A UN spokesman, Christopher Gunness, said the list had been compiled from the names released so far.

It includes eight teenagers who were killed on Saturday afternoon as they were leaving the UN compound in Gaza where they had been attending a vocational training course.

"We are demanding a full and transparent investigation," Mr Gunness said.

A local UN staff member was killed over the weekend when an Israeli missile hit a police station that the man had been visiting with a family member.

"We remain very concerned at the high number of civilian casualties being caused by this operation," Mr Gunness said.

Captain Rutland said that responsibility for any civilian deaths or injuries lay solely with Hamas "which cynically places its military installations in civilians areas".

Living conditions inside Gaza continued to worsen, with severe shortages of medical supplies.

The Israeli bombardment of Gaza's border with Egypt has put a halt to smuggling operations that had been a major source of supply for food, fuel and other daily needs.

Witnesses at the Rafah border crossing described a chaotic scene as young men tried to force their way into Egypt amid sporadic exchanges of gunfire between Hamas and Egyptian forces. Egyptian state television reported that one Egyptian border guard was fatally shot by a Hamas gunman. A Palestinian man was killed by an Egyptian guard near Rafah.

Israel allowed four trucks carrying urgently needed humanitarian supplies to pass through. Israeli officials said they will open the crossing points again today in order to allow the entry of further humanitarian aid.

Palestinians fired about 30 rockets at Israel on Sunday, significantly lower than the 90 rockets fired on Saturday. This was far fewer than IDF intelligence estimates had predicted.

Since the beginning of the offensive, Israeli police have arrested about 100 rioters in East Jerusalem and in Arab villages.