RIYADH (AFP) - Saudi-led warplanes struck key targets of Yemeni Houthi rebels in their northern stronghold of Saada, state media said Friday after Riyadh vowed "harsh" punishment for deadly cross-border bombardments.

The raids on Thursday targeted two control centres, a communication complex and a landmine factory, the kingdom's SPA official news agency said.

The Saudi-led coalition, which has waged six weeks of air strikes in support of exiled President Abedrabbo Mansour Hadi, also destroyed other rebel positions in Saada province, which borders Saudi Arabia, SPA said.

Saudi Arabia warned Thursday that the Iran-backed Shi'ite Houthis had crossed a "red line" in shelling populated border areas in the kingdom.

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

Saada would be among the targets in the coalition's sights, he said, speaking just hours after the kingdom proposed a humanitarian ceasefire.