Huthi militias in Yemen have crossed a "red line" and will pay a high price for their deadly bombardments of Saudi Arabia, the Saudi-led coalition said on Thursday.

"The equation is different, the confrontation is different, and they will pay a harsh and expensive price," coalition spokesman Brigadier General Ahmed al-Assiri told reporters.

"The safety of Saudi Arabia is a top priority for the coalition and the Saudi armed forces. It is a red line they crossed."

Shells fired from Yemen killed five people in the kingdom's Najran region on Wednesday in a second day of cross-border bombing blamed on the Iran-backed Shiite Huthi rebels, civil defence authorities said.

Three others died in a mortar and rocket attack on Najran on Tuesday, the first time the Huthi rebels attacked a Saudi community since the Saudi-led coalition on March 26 began air strikes against the Huthis and their allies in neighbouring Yemen.

In Jazan district adjacent to Najran, a missile killed a husband and wife on Tuesday, the civil defence department said.

It was "killing for the sake of killing," Assiri said.

State television on Thursday reported further strikes but said they landed in an uninhabited area.

"Now the confrontation is targeting Saudi Arabia's borders, targeting Saudi citizens, targeting the safety and security of Saudi cities. Here we confirm that this is unacceptable," the spokesman added.

He said an operation to respond "starts now" and will target "all the leaders of the organisation, the areas they gather, their leadership locations, and the planners. And it will not be a limited operation."

The Huthi stronghold of Saada, in northern Yemen, will be among the areas in the coalition's sights, Assiri said.

Hours earlier, Saudi Foreign Minister Adel al-Jubeir proposed a five-day ceasefire to allow aid into Yemen but said the Huthis would have to abide by it.

The coalition took action in March after Riyadh feared the Huthis would take over all of Yemen and pull it into the orbit of the kingdom's Shiite regional rival Iran.
Source: AFP