Isreal has dismissed calls for a 48-hour halt to its Gaza offensive as unrealistic. But officials rejected the idea as unrealistic and vowed to continue its assault on the strife-torn region. And they have warned that the threat of a ground offensive remains if the Hamas attacks continue. Thousands of ground troops, backed by tanks and artillery, are in position along the border and Israeli aircraft have kept up a relentless string of airstrikes with the most recent raids smashing a government complex, security installations and the home of a top militant commander. The offensive was launched on Saturday after the Islamic movement Hamas defied Israel's warnings that it would not stand for the rocket attacks on the south of the country that resumed nearly two months ago, toward the end of a recently expired ceasefire. Israeli Prime Minister Ehud Olmert told President Shimon Peres that the aerial phase of the operation was the first of several stages that had been approved. Almost 370 Palestinians have been killed, most of them members of Hamas security forces, but at least 64 of them civilians, according to UN figures. More than 1,400 people have been injured in the attacks. Hamas officials have declared they will never accept a truce with Israel "under these circumstances". Spokesman Fawzi Barhoum said talks to stop the fighting and enter an agreement would be "a matter of bargaining between the victim and the executioner." The Quartet of Mideast peacemakers - the United States, the United Nations, the European Union and Russia - has called on Israel and Hamas to implement an immediate cease-fire in Gaza and southern Israel. Mr Barhoum called on Arab and Islamic countries "to unite and stop this aggression, lift the siege, open the crossings and rebuild Gaza." Meanwhile, Prime Minister Gordon Brown has called for an urgent ceasefire amid the "humanitarian crisis" in Gaza. Mr Brown said: "It is vital that moderation must now prevail - there's a humanitarian crisis." Britain is donating £5 million towards humanitarian aid in the crisis-stricken region. A spokesman for the US President said: "President Bush thinks that Hamas needs to stop firing rockets and that is what will be the first steps in a ceasefire.