Israeli troops killed two Palestinian teenagers and a gunman in the Gaza Strip on Thursday as the Israeli army pressed ahead with its biggest and bloodiest offensive in Gaza for four years. Palestinian medics said two teenagers, both 15, were on their way to school when they were hit by a shell. Israeli military officials said the two were killed by a helicopter missile while attempting to launch a Qassam rocket. Israeli forces later exchanged fire with two gunmen trying to plant bombs near a Jewish settlement in southern Gaza, killing one of them, the military said. The other escaped. The deaths raised to at least 79 the number of Palestinians killed since Israel launched the offensive eight days ago after two toddlers were killed in a rocket strike on an Israeli town. Palestinian sources said at least 46 of the dead were militants and the rest were believed to be civilians. Three Israelis and a Thai labourer have also been killed. Despite growing U.S. and international pressure, Prime Minister Ariel Sharon ordered the army to press ahead with the operation to halt rocket attacks on Israelis ahead of a planned pullout from Gaza next year. --More 2352 Local Time 2052 GMT