Avi Pazner, an Israeli government spokesman, said on Friday that the draft of a United Nations resolution on ending the Lebanon conflict is "unacceptable in its current state".
"The resolution draft was altered on the request of Lebanon, which is under pressure from Hezbollah. The discussions have now taken the appearance of haggling at a Lebanese carpet shop - we will not fall into this trap," Pazner said.
"The diplomatic option having failed for the moment, we did not have any other choice but to lean towards the military option," he said.
The orders to sweep to the Litani River, 20km from the border, had been approved by Ehud Olmert's security cabinet on Wednesday but put on hold.
Olmert's go-ahead on Friday came shortly after US and British ambassadors to the UN said the Security Council was on the verge of a deal aimed at halting the month-old war.
But Qatar's UN ambassador, Nassir Abdulaziz al-Nasser, said changes were still being discussed with Lebanese and Israeli authorities.
Aljazeera's correspondent in New York said the amended resolution would include a call for the strengthening of the UN Interim Force in Lebanon instead of sending a new international force.
Dan Gillerman, Israel's UN ambassador, said Israel was still engaged in negotiations over a UN resolution despite Olmert's order.
"The Americans are aware of what we can accept and cannot accept. And I very much hope that in the end of the day there will not be submitted a draft that is unacceptable to us," Gillerman told Israel's Channel Two television.
Olmert faced a backlash on Friday over the draft UN resolution, with army officers charging that they were held back and right-wing rivals calling for new elections.
"The Americans are aware of what we can accept and cannot accept. And I very much hope that in the end of the day there will not be submitted a draft that is unacceptable to us"
Israel's ambassador to the UN
Opinion polls, taken before details of the proposed Security Council resolution emerged, showed eroding public support in Israel for Olmert, a career politician who lacks the combat credentials of many of his predecessors.
Only 20% of those surveyed by Haaretz thought Israel was winning the war.
In contrast, a senior Lebanese political source said Lebanese leaders had made "major progress" in talks with a top US official on Friday on the UN resolution.
"We are discussing the details. There is serious and major progress that could lead to an understanding in the next few hours," the source said. "There are no more basic sticking points."
David Welch, the US assistant secretary of state who has been shuttling between Beirut and Jerusalem, held separate talks in the Lebanese capital with Fouad Siniora, the prime minister, and Nabih Berri, the Shia parliament speaker who has negotiated on Hezbollah's behalf.