Israel, which declared war against Islamist militant group Hamas following an unprecedented attack by the group in Israeli territory, has instructed Chevron Corp. to shut in the Tamar natural gas platform offshore the northern coast. 

Alternative fuels are to be used as necessary to ensure the economy can operate, Israeli officials said.

Israel had formally declared war on Hamas and was ramping up a major military operation in Gaza following the deadly attack on Saturday. More than 1,000 people had been killed and dozens were missing as of early Monday. At least nine Americans were killed in the attack, and other U.S. citizens were reportedly missing, the U.S. State Department confirmed.

“As reported by the Tamar working interest owners to the Israel Securities Authority,...