By Simon Lewis and Tuvan Gumrukcu ISTANBUL (Reuters) -U.S. Secretary of State Antony Blinken was meeting the leaders of Turkey and Greece on Saturday at the start of a week-long trip aimed at calming tensions that have spiked across the Middle East since Israel’s war with Hamas began in October. The Biden administration’s most senior diplomat began in Istanbul, meeting Turkish President Tayyip Erdogan, a strong critic of Israel’s military actions in Gaza. In the talks, Blinken “emphasized the need to prevent the conflict from spreading, secure the release of hostages, expand humanitarian assis…