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By Nidal al-Mughrabi and Jeffrey Heller GAZA/JERUSALEM (Reuters) - A Gaza ceasefire was in jeopardy just hours after it began on Friday, with the Islamist group Hamas saying Israeli tank fire killed four people and Israel accusing militants of violating the truce. The 72-hour break announced in a joint statement by U.S. Secretary of State John Kerry and U.N. Secretary-General Ban Ki-moon was the most ambitious attempt so far to end more than three weeks of fighting, and followed mounting international alarm over a rising Palestinian civilian death toll. Israel launched its offensive in Hamas Islamist-dominated Gaza on July 8, unleashing air and naval bombardments in response to a surge of cross-border rocket attacks. Three civilians have been killed by Palestinian rockets in Israel.