Israel rejected mounting international pressure for a pause in its barrage of Gaza and stepped up preparations yesterday for a ground offensive.

Hamas said that its fighters had had their first skirmish with Israeli ground troops in the southern Gaza Strip. The claims of a clash with a unit of special forces were denied by the Israeli army.

Israeli bombers and artillery pounded buildings for a fifth straight day, as Palestinian militants continued to fire rockets ever deeper into the Jewish state. Almost 400 Palestinians have been killed, a quarter of them civilians, according to the United Nations.

Israel has authorised the call-up of 2,000 more reservists in preparation for a large-scale incursion.

France called for a “humanitarian ceasefire” to allow aid into Gaza, where food stocks are dwindling and medical necessities have all but run out. Gordon Brown added to the international calls for a truce. “It is vital that moderation must now prevail – there’s a humanitarian crisis,” he said.

A Palestinian aid worker said some families were foraging for wild plants to cook after the bombing led to the suspension of UN supplies. Israel has allowed some aid in, but Palestinian officials said it was not enough.

Mark Regev, a spokesman for Ehud Olmert, Israel’s Prime Minister, said: “Giving Hamas a respite just to regroup, rearm, is a mistake. The pressure on the Hamas military machine must continue.”

A Hamas spokesman in Gaza said that his movement had received no truce overtures. “If such a proposition is made to us, we will examine it as we are favourable to any initiative that will put an end to the aggression and totally lift the blockade,” he said.

Senior Israeli officials have said that the aim of Operation Cast Lead is to obliterate the leadership of Hamas and have warned that the campaign could take weeks. So far, the operation has failed to achieve even its basic goal of halting Hamas rocket fire, with Chinese-made Grad missiles and a new type of Iranian-supplied Fajr-3 rocket penetrating ever farther into Israel. The town of Beersheba, 25 miles from the Gaza border, has been struck for the first time by at least five new rockets.

But Yuval Diskin, the head of Israel’s internal security forces, said that Hamas’s ability to rule the territory had been “badly impaired” and rocket factories had been wiped out. Bombardments along the southern border with Egypt had also hampered the Islamists’ ability to smuggle in fresh munitions through their extensive tunnel network, he said.

Military sources said that Israel was still broadening its range of targets, hitting a money-changer responsible for transferring funds to Hamas.

There were signs of disagreement at the top levels of the Israeli command, with Ehud Barak, the Defence Minister, reportedly favouring a lull and Mr Olmert and the army’s top brass wanting to press ahead. Preparations for an invasion have been completed, Israeli media reported, though poor weather could delay any military advance.

Many commentators worry that a ground invasion will be costly both to Israel and the Palestinians, with determined Hamas fighters well dug in among the civilian population and ready to fight to the death. Israeli officials said that they were not entirely closed to a truce but it had to be a lasting one to which Hamas was properly committed.

Such a truce will be difficult for mediators, led by Egypt, to arrange as both sides attach peripheral conditions. Hamas wants the border crossings to be opened, which Israel will only agree to if they are manned on the Palestinian side by Fatah, Hamas’s bitter rival, with whom Israel is in slow-moving peace talks. Israel, on the other hand, considers Hamas’s weapons smuggling a breach of any truce.

An emergency meeting of the Arab League in Cairo called on Hamas and Fatah to overcome their rift and form a unity government.

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