Iranian security officials helped plan Hamas’s Saturday surprise attack on Israel and gave the green light for the assault at a meeting in Beirut last Monday, according to senior members of Hamas and Hezbollah, another Iran-backed militant group.

Officers of Iran’s Islamic Revolutionary Guard Corps had worked with Hamas since August to devise the air, land and sea incursions—the most significant breach of Israel’s borders since the 1973 Yom Kippur War—those people said.

Details of the operation were refined during several meetings in Beirut attended by IRGC officers and representatives of four Iran-backed militant groups, including Hamas, which holds power in Gaza, and Hezbollah, a Shiite militant group and political faction in Lebanon, they said.