JERUSALEM (AFP) - Israel threatened on Sunday to launch a major offensive against the Hamas-ruled Gaza Strip as violence simmered around the impoverished territory days after the end of a truce with the movement. The two frontrunners in the race to become prime minister after a snap election in February both vowed to topple Hamas, which has run Gaza since violently seizing power there in June 2007. "Israel must topple the Hamas rule in Gaza and a government under my command will do just that," Foreign Minister Tzipi Livni, who leads the Kadima party that heads the government coalition, was quoted as saying by Israeli media. "Israel must react when it is fired upon, must re-establish its force of dissuasion and stop the rockets." Benjamin Netanyahu, the leader of the right-wing Likud party, which is currently leading opinion polls ahead of the February vote, echoed the sentiment. "In the long run, we have no choice but to topple Hamas rule," he was quoted as saying by the Ynet news website as he toured the southern Israeli town of Sderot, which has borne the brunt of rocket attacks from Gaza. "Right now we have to go from passive response to active assault." Senior defence officials said after the weekly cabinet meeting that Israel was preparing to respond to continuing rocket strikes from the besieged Palestinian coastal strip. "We are preparing our response to the Hamas threat, with the decision yet to be taken on the timing and the scale," Amos Gilad, a senior adviser to Defence Minister Ehud Barak, told public radio. Earlier, a senior Israeli defence official told AFP that a major military confrontation in the territory was unavoidable after the Gaza fighters again targeted southern Israel with rockets. "It is obvious where we are heading in Gaza. The situation is intolerable but clear. The army's considerations are the only thing that is deciding when events will unfold," the official said on condition of anonymity. Palestinian witnesses said that on Sunday an Israeli infantry unit backed up by three tanks was operating several hundred metres (yards) inside Palestinian territory in southern Gaza and had arrested a farmer. An army spokesman denied there was an operation, saying soldiers conducting maintenance work on the Israeli side of the border shot back after coming under fire and did not report hitting anyone. Tensions around Gaza have risen steadily since Friday, when Hamas said it would not renew a six-month truce with Israel that came into effect in June after months of Egyptian mediation. "The politicians aren't in any rush to reach election day with an incomplete military operation and only partial results hanging around their necks," wrote military analyst Alex Fishman in top-selling daily Yediot Aharonot. "And worse than that, to be accused of having ordered a military operation just to improve their chances at the ballot box," he said.