PARIS (AP) -- Saudi Arabia declared a unilateral cease-fire in Yemen that would start May 12 and urged Shiite rebels and their allies to stop fighting. It was unclear if the Iran-backed Houthis were prepared to lay down their arms.

At a news conference Friday with U.S. Secretary of State John Kerry, Saudi Foreign Minister Adel al-Jubeir said the halt in fighting would start next Tuesday at 11 p.m. local time. The so-called humanitarian pause is renewable, depending on compliance by the rebels who have chased Yemen's internationally recognized government out of the country.

The announcement in Paris came hours after the Saudi-led coalition carrying out airstrikes in Yemen declared a rebel stronghold along the kingdom's border a "military target" and gave residents an ultimatum to leave the region by nightfall. The ultimatum was reported by state TV.

The escalation in the northern Yemeni province of Saada came in response to recent cross-border attacks by the Houthis on Saudi cities near the frontier, to which the coalition has vowed a "harsh response."

Yemeni officials said that more than 50 airstrikes hit Saada overnight and in the early hours in the morning. The Saudi Press Agency reported that warplanes destroyed a land-mine factory, a telecommunications complex and command centers in Saada.

Al-Jubeir and Kerry said they were working on a cease-fire Thursday in Riyadh but that they still needed more time to flesh out the details. On Friday in Paris, where they gathered with other Arab foreign ministers, Kerry said the cease-fire is conditioned on no bombing, no shooting, no repositioning of troops and no movement of heavy weapons.

"A humanitarian catastrophe is building," Kerry warned, saying civilians were running out of food, fuel and medicine and that aid groups needed to be allowed to get supplies into and around the country.

He said anyone who cares about Yemen, or even pretends to care, would act to help put the cease-fire in place.

By The Associated Press