Gaza City: Israeli forces unleashed a wave of strikes across the Gaza Strip on Friday after saying its troops came under fire, killing three Hamas members as fears grew of a wider conflict.Fireballs exploded into the sky over the Palestinian enclave and Israel reported projectiles had been fired back at its territory, prompting the UN to urge all sides to step "back from the brink" after months of tensions.A fourth Palestinian was also shot dead during protests along the frontier with Israel, Gaza´s health ministry said.Israeli Defence Avigdor Lieberman warned of a "much tougher" response to any fresh missile fire by Hamas after the army reported "three launches" from the territory, two of which were intercepted.The United Nations envoy for the Middle East peace process, Nickolay Mladenov, said "everyone in Gaza needs to step back from the brink. Not next week. Not tomorrow. Right NOW!""Those who want to provoke Palestinians and Israelis int...