Israeli helicopters fired missiles at a group of armed men sitting near the Gaza City beach Sunday, killing four Hamas fighters, including a fugitive commander, just hours after Israel's army chief said Hamas members were targets for "liquidation." The attack occurred just 200 yards from the office of Palestinian security chief Mohammed Dahlan. Palestinian officials said Sunday's attack would undermine a planned Palestinian security clampdown that began Saturday with moves against arms smugglers, casting fresh doubt on an already shaky peace plan. "This aims to sabotage the efforts that began last night," said Saeb Erakat, a senior Palestinian lawmaker. "It's very obvious that the Israeli government is acting as if the Palestinian Authority is something from the past." One witness, Shadi Wassi, said he was about to enter his house "when suddenly a huge explosion shook the ground under my feet. When I looked back, I saw a big flame burning the trees, then another two huge explosions hit the area." Other witnesses said the men were sitting near the beach for about half an hour when the missiles hit. Bystanders carried the bloodied body of one man to an ambulance, as the helicopters fired flares. Hamas identified the dead men as fighters Ahmed Aishtawi, Wahid Hamas, Ahmad Aub Helal and Mohammed Abu Lubda. The military strike came as Palestinian leaders were locked in a power struggle over command of their security forces. Hamas remained defiant after Sunday's missile strike. "If the Israelis thought assassinations would destroy our determination to continue in our resistance, to continue defending ourselves, they are mistaken," Hamas spokesman Ismail Haniya said. "We will move ahead whatever the sacrifice." Also Sunday, a rocket fired from the Gaza Strip landed about four miles from the Israeli city of Ashkelon, about 10 yards from an unmanned lifeguard post. It was the deepest a Palestinian rocket has struck in Israel in recent memory, the Jewish army said. **PHOTO CAPTION*** Palestinians carry the body of an unidentified man killed in an Israeli helicopter missile strike on a car in west Gaza City. (AFP/Mohammed Abed)