40,000 IDF troops and reservists were massed along the northern border Wednesday evening in preparation for Israel's largest and deepest ground incursion into southern Lebanon since the beginning of Operation Change of Direction last month. Some 7,000 IDF troops were operating in southern Lebanon Wednesday, clashing with Hizbullah guerrillas in several villages while holding and maintaining position along a 10-kilometer-deep security zone the IDF had created. A high-ranking IDF officer and member of the General Staff told The Jerusalem Post Wednesday that it would take the military at least one week to reach the Litani and beyond, and to set up position and begin taking control of the area. The officer said that it would then take four to six weeks to clear out southern Lebanon, from the Litani river south, of the Hizbullah presence and to destroy the thousands of Katyusha rockets and rocket launchers believed to be in that area. The IDF estimates that the area between the Litani river and the security zone that military forces are currently maintaining is home to 70 percent of the Katyusha rockets launched at northern Israel. In the plan approved by the cabinet on Wednesday, the IDF was also granted permission to cross the Litani river into areas like Nabatiyeh, from where Hizbullah has been firing rockets at the upper Galilee, as well as Haifa and other coast-lying cities. Hizbullah's Nasser Unit, in charge of southern Lebanon, was still operational, the high-ranking officer said, and numbered several thousand Hizbullah fighters, including reservists, which the guerrilla group had called up in anticipation of Israel's planned massive ground incursion. Hizbullah, the officer said, still retained its command and control abilities throughout Lebanon and had fighters deployed in between 100-130 villages from the Litani south. North of the river, the officer said, Hizbullah had a smaller presence, but one that still numbered several thousand fighters. In southern Lebanon, within the security zone the IDF had created, soldiers were still battling in two Hizbullah strongholds - Bint Jbail and Ayta a-Shaab, scenes of heavy fighting between IDF troops and Hizbullah gunmen on Wednesday.