SHAAR YISHUV, Israel (REUTER) - Israeli cabinet ministers observed a minute of silence and proclaimed a day of national mourning on Wednesday for 73 soldiers killed in the worst air disaster in Israeli military history. Prime Minister Benjamin Netanyahu said the deaths in a crash of two military helicopters heading for Lebanon on Tuesday evening were part of Israel`s war for survival. "We mustn`t forget for even a moment if our people want to live we must win the hard war imposed on us by those who refuse in their blind hate to make peace with us and continue to want to destroy us," he told parliament at a special meeting broadcast live on Israeli television and radio. Two U.S.-made Sikorsky CH-53 transporters collided and crashed around 7 p.m. (1700 GMT) on Tuesday in Israel near the Lebanese border. The helicopters were flying to a 15 km (nine mile)-wide occupation zone in south Lebanon which Israel carved out in 1985 to repel guerrilla attacks. The army said 13 officers were among the soldiers killed in what it called the worst air disaster in Israeli military history. Israel`s defence minister appointed a commission to investigate. "There were no survivors. Everyone who was in the helicopters was killed," army chief Lieutenant-General Amnon Shahak told reporters. "Apparently there was (physical) contact between the helicopters in the area. The reason is unclear," air force chief Major-General Eitan Ben-Eliahu told a news conference. "We know of no malfunction before the incident," he added. Israelis, including politicians from left to right, mourned the loss of life. Most of the dead were around the age of 20. King Hussein, Arafat and Egypt`s President Hosni Mubarak - all partners to peace deals with Israel - sent condolences. The question of Israel`s involvement in Lebanon came up again this week following the killing of three soldiers in a Hizbollah bomb attack.Twenty-seven Israeli soldiers were killed fighting guerrillas in Lebanon last year. Mordechai appointed a five-man committee headed by former air force chief David Ivri to launch an investigation on Wednesday into the disaster. In Israel`s previous worst military air disaster, 54 paratroopers were killed when a Sikorsky crashed in 1977.