Osama bin Laden has called for Muslims to wage a holy war against Israel over the invasion of Gaza. The al-Qaeda leader called for jihad in an audio tape that appeared on Islamist websites. In response, the US said it demonstrated his isolation and was likely an effort to raise money. The news came as United Nations Secretary-General Ban Ki Moon visited the Middle East to press for an immediate ceasefire in the Gaza Strip. More than 900 Palestinians have been killed since the Israeli conflict with Hamas began. A UN Security Council resolution adopted last Thursday calls for a truce, a full withdrawal of Israeli forces and measures to prevent arms smuggling and reopen all border crossings into Gaza. Mr Ban is in Cairo for talks with Egyptian President Hosni Mubarak, who launched an initiative with France a week ago aimed at achieving a temporary halt to the fighting followed by a permanent cease-fire and arrangements on border security. Israel has tightened its grip on the Palestinian enclave but insists "there is still work" ahead after tanks rolled into densely populated parts of Gaza City as the fight with Hamas intensified. Islamist militants fought with Israeli forces backed by helicopter gunships and naval fire in what appeared to be the most ferocious fighting since Israel sent ground troops into the Palestinian enclave ten days ago. Warplanes continue to pound the southern Gaza town of Rafah, near the coastal territory's border with Egypt. They attacked 60 targets, including tunnels used by Gaza militants to smuggle arms across the border from Egypt, weapons-making sites and Hamas command posts, the army said. Two rockets hit Beersheba in southern Israel, causing no casualties. There have also been reports of rockets being fired into northern Israel by militant groups in Lebanon.