Israel says "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. On the 18th day of the offensive, Israeli forces tightened their hold around the city and as explosions and heavy machinegun fire echoed, Israeli tanks drew closer to its densely-populated centre but did not enter. Meanwhile, warplanes continued 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. Medical workers said 18 Palestinian gunmen, most of them Hamas members, and three civilians were killed in fighting in the territory on Tuesday. At least 933 people in the Gaza Strip have been killed and nearly 4,000 wounded since December 27. The health minister in Gaza's Hamas-run government said close to 400 of those were women and children. Thirteen Israelis - ten soldiers and three civilians hit by mortar bombs and rockets from the Gaza Strip - have been killed. Lieutenant-General Gabi Ashkenazi, chief of staff of Israel's armed forces, said: "We have achieved a lot in hitting Hamas and its infrastructure, its rule and its armed wing, but there is still work ahead. "We are working to deepen the blow to its military arm, reduce (Hamas) fire, strengthen (Israeli) deterrence and improve the security situation for residents of southern Israel living under the threat of (rocket) attacks." Israel has rebuffed as "unworkable" a UN Security Council ceasefire resolution last week and said a truce must ensure Hamas cannot re-arm through tunnels under the Gaza-Egypt border in an area known as the Philadelphi corridor. United Nations Secretary-General Ban Ki Moon, in the region to press for a truce in a week of talks with leaders in Egypt, Israel, Jordan and Syria, said: "My message is simple, direct, and to the point: the fighting must stop. To both sides, I say: Just stop now." US President George W Bush said Israel has a right to defend itself but he hopes it will "continue to be mindful of innocent folks" and continue to allow the delivery of humanitarian aid to Palestinian civilians.