JERUSALEM (Reuters) - Israel has agreed to extend by four hours a humanitarian truce agreed with Hamas and other armed groups in the Gaza Strip on Saturday, an Israeli government source said, declining to be named. It was **t immediately clear if Hamas Islamists, who control Gaza, had also agreed to prolong the original 12-hour ceasefire and continue the quiet until midnight (2100 GMT). (Writing by Maayan Lubell; Editing by Crispian Balmer)