GAZA CITY (AFP) - Israeli tanks rolled into Gaza on Saturday and engaged in night-time battles with Hamas forces after more than a week of airstrikes that left hundreds of Palestinians dead and widespread destruction. The Israeli army said "large number" of troops had moved into Gaza and the government called up thousands of reservists to join the ground offensive aimed at ending rocket and mortar attacks by Hamas and its militant allies. Tanks opened up fire after crossing the border and casualties were reported around Gaza City and the northern towns of Beit Lahiya and Jabaliya. Hamas forces fired back using mortars and rockets, witnesses said. Gaza emergency services said one child was killed and 11 other children were wounded. They said two other Palestinians, including a woman, were killed east of Gaza City. The Hamas television station said Israeli troops had been killed, but there was no immediate confirmation of this. Israel jammed the Hamas broadcasts, posting a caption reading: "Hamas leadership, your time is over." In a statement, Prime Minister Ehud Olmert's office said the army would take over sites from which militants have fired rockets at Israel and deal a "hard blow" to the Islamists. "Large number of forces are taking part in this stage of the operation including infantry, tanks, engineering forces, artillery and intelligence," a military spokesman said. The military said a 20-nautical-mile naval blockade would be enforced along the coast. Thousands of Israeli troops and tanks had encircled Gaza as artillery lobbed shells ahead of the offensive. F-16 jets roared over the enclave where at least 460 people have been killed in eight days of air attacks. Militant rocket fire from Gaza into Israel on Saturday slightly wounded three people, and the exiled Hamas leader threatened a "black destiny" in the event of a ground invasion. Hamas vowed on Saturday to turn the Gaza Strip into a "cemetery" for the Israeli army. "Your incursion into Gaza will not be a walk in the park and Gaza will become your cemetery God willing," Hamas spokesman Ismail Radwan said reading a statement on Hamas television. "You do not have any other choice but to unconditionally end the aggression and lift the blockade," he said. A spokeswoman for president-elect Barack Obama declined to comment Saturday on Israel's ground offensive into Gaza, saying only the next US president was closely following events. "The president-elect is closely monitoring global events, including the situation in Gaza," his national security spokeswoman Brooke Anderson said in a statement. But she offered no comment on the violence in Gaza and, using a phrase repeated often by Obama and his aides, said: "There is one president at a time and we intend to respect that." The head of the Lebanese group Hezbollah urged Hamas to inflict huge losses on Israel whose troops entered the Gaza Strip on Saturday after eight days of pounding the enclave by air and sea. "Our brothers in the resistance in Palestine know that it is by inflicting the biggest possible losses on the Israeli enemy during the ground confrontation that they will win the battle," Hassan Nasrallah said. "It is when the resistance kills soldiers and destroys tanks that the course of the battle will be determined," he added. Israeli military operations in Gaza will last "many days," the army said after ground troops moved into the Hamas-run enclave. "The combat will last many days," said a statement. Israel is ready for any development on its border with Lebanon, Defence Minister Ehud Barak said on Saturday in a veiled warning to Hezbollah as the Jewish state launched a ground offensive in Gaza. In one of the deadliest single strikes of the campaign so far, an Israeli jet hit a mosque in the northern town of Jabaliya, killing at least 16 people among more than 200 people at prayer, medics and witnesses said. Four children " two brothers aged 10 and 12 and two cousins under 16 " were among the dead at the Ibrahim al-Maqadna mosque, medics said. Dozens of wounded were dragged from the rubble. Two Hamas military commanders were also killed in airstrikes as pressure mounted on the Islamists. Their deaths came just two days after an Israeli missile killed top Hamas leader Nizar Rayyan. Earlier a school in northern Gaza was destroyed in an airstrike and a guard was killed. Israel said it had been used for firing rockets. Four people have been killed in Israel by more than 500 rockets fired from Gaza over the same period. Exiled Hamas leader Khaled Meshaal warned Israel on Friday of a "black destiny" if it invaded. But Washington gave its key Middle East ally free rein to launch a ground operation. In his weekly radio address, President George W. Bush called on Hamas "to turn away from terror" and rejected calls for a unilateral ceasefire that he said would allow the Islamists to continue hitting Israel with rockets. Palestinian president Mahmud Abbas was to meet French President counterpart Nicolas Sarkozy in Ramallah on Monday before travelling to New York to appeal for a ceasefire before the UN Security Council. Jordan's King Abdullah II told Quartet Middle East peace envoy Tony Blair that the world's "silence" on the humanitarian crisis in Gaza was unacceptable.