By Nidal al-Mughrabi, Bassam Masoud and Dan Williams CAIRO/GAZA/JERUSALEM (Reuters) – The U.S. said “very serious” negotiations were taking place on a new Gaza ceasefire and release of more Israeli hostages, but prospects for a deal remained uncertain as Hamas insisted it would not discuss anything less than a complete end to Israel’s offensive in the Palestinian enclave. Hamas leader Ismail Haniyeh visited Egypt on Wednesday for the first time in more than a month for discussions with Egyptian officials who are seeking to mediate another truce. A source briefed on the negotiations said envoys…