Tehran, Iran - Yemen's Iran-backed Houthi rebels joined the month-old Middle East war, claiming two missile attacks on Israel that raised concern on Sunday about the war spreading to the Red Sea.

Fears of a widening conflict came as the Washington Post reported that the Pentagon was preparing plans for weeks of US ground operations in Iran – though it said President Donald Trump has yet to approve any deployment.

The involvement of Houthi rebels has added a new complexity to a conflict that has already impacted a wide swathe of the Middle East.

During Israel's recent war in Gaza, the Houthis – claiming solidarity with Palestinians – attacked shipping in the Red Sea and Gulf of Aden, forcing companies to take costly detours.

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Until Saturday, they had sat out the latest conflict, even as the Red Sea grew more vital.

A Houthi spokesman on Saturday said the group fired "a barrage of cruise missiles and drones targeting several vital and military sites" in Israel.

Since the launch of hostilities, Saudi Arabia has rerouted much of its oil exports via the Red Sea to avoid the Strait of Hormuz, which Iran says it has closed to shipping from hostile powers.