IDF Chief of General Staff Lt.-Gen. Dan Halutz said Thursday that an offensive in Lebanon would not end until Israel's security was restored, and vowed to destroy Hizbullah's arsenal and military capabilities. "The fighting in the north ... could last a long time," Halutz said in a letter to soldiers and officers. "We are being tested at this time. Our moral strength and value will reflect on the state of Israel and its residents and on their ability to continue to stand up to the threat on the front." "We will operate for as long as necessary until security is returned to the state of Israel," he added. Sheikh Hassan Nasrallah, in the Bourj al-Barajneh refugee camp in southeast Beirut. It was still unclear who was in the bunker at the time and what their fate was, but IDF sources said the bunker was totally destroyed and that all that was left was a crater. The IDF obtained intelligence information late Wednesday night that Hizbullah leaders possibly including Nasrallah had taken refuge inside the bunker. A wave of aircraft immediately took to the air and dropped 23 tons of explosives on the bunker. IDF sources would not confirm that Nasrallah was in the bunker at the time, but said that high-ranking Hizbullah leaders were inside, and that it appeared that the attack was successful. Hizbullah has said none of its "leaders or members" died in the IAF strike. "The truth is that the building targeted by the enemy warplanes with 23 tons of explosives is just a building under construction to be a mosque for prayers," said the statement, issued on the group's Al-Manar TV and faxed to The Associated Press. "It seems that the enemy wants to cover up its military and security failures with lies and claims of imaginary achievements," it said. The IDF said the strike occurred between 8 and 9 pm but refused to give further details. Reporters in Beirut said they heard a huge explosion around 8:30 p.m. Hizbullah has a headquarters compound in Bourj al-Barajneh that is off limits to the Lebanese police and army, so security officials could not confirm the strike. Despite the airing of Hizbullah's claims that the IAF had hit a mosque under construction, the IDF Spokesman's office insisted to The Jerusalem Post early Thursday morning that the IAF had hit a Hizbullah bunker. Also early Thursday morning, Israel's UN Ambassador Gillerman said in a CNN interview that "I can assure you that we know exactly what we hit. ... This was no religious site. This was indeed the headquarters of the Hizbullah leadership." Since the IDF went to war with Lebanon last Wednesday, fighter jets have repeatedly bombed another bunker in the Dahiya neighborhood in Beirut, also said to be the main nerve center and headquarters of Hizbullah. The IAF has so far carried over 3,000 sorties over Lebanon, and in the past day attacked 200 targets throughout the country, including Hizbullah headquarters, cars carrying terrorists, Katyusha launchers and weapons warehouses.