The Arab coalition carried out air strikes in Yemen's Saada, a bastion of Houthi rebels, and warned all civilians to leave the restive province after declaring the entire territory a "military target".

Saudi state television channel Al Ekhbariya said on Friday that the region would become a target, as General Ahmed al-Asiri, the military spokesman for the coalition, said leaflets had been dropped in the Old Saada district urging residents to leave by 19:00 local time (1600 GMT) on Friday.

The escalation comes in response to recent cross border attacks by the Houthis, which targeted Saudi cities near the Saudi-Yemen frontier.

"Our work now is reaching those (Houthis) who planned these attacks and who are hiding in Saada, and the places where the militias are," Asiri said.

"Our military operation will be longer and harsher, and will go after all Houthi commanders," he added.

Al Jazeera's Mohamed Vall, reporting from Riyadh, said the announcement was the first in which the coalition declared it was going after the entire Houthi leadership.

Doctors Without Borders criticised the announcement, saying that the bombing of Saada could lead to "massive loss of life", as many people may not be able to leave.

"The bombing of civilian targets, with or without warning, is a serious violation of International Humanitarian Law. It is even more serious to target a whole province," Llanos Ortiz, Doctors Without Borders' emergency coordinator in Hajjah province, said.

"It is impossible for the entire population of Saada province to leave within hours. Many people have no transport or fuel due to the coalition's blockade. Many others have no access to information as the province's phone networks are barely operational."

Mortar and rocket salvoes fired from Saada killed eight people in the Saudi city of Najran n Tuesday and Wednesday, and Houthi forces shelled a Saudi air defence site near Najran on Thursday.

In response, the coalition launched more than 50 air raids in Saada overnight, destroying a suspected land-mine factory, a telecommunications complex and command centers.

Humanitarian pause

Riyadh on Thursday proposed a five-day humanitarian ceasefire to help civilians stricken by food, medicine and fuel shortages, on condition the Houthis stop fighting.

Foreign Minister Adel al-Jubeir said at a meeting of Gulf ministers in Paris on Friday that the ceasefire would start on May 12, but would only hold if the Houthis complied with the rules.

Jubeir said the air strikes on Saada were not tied to the ceasefire deal, saying he hoped the Houthis would "come to their senses" and agree to the truce.

"This is an opportunity for the Houthis to look out for the well-being of the Yemeni people," Jubeir said. "This is entirely in their hands."

Relief agencies say they desperately need supplies, including fuel to run infrastructure such as hospitals.

John Kerry, who was also at the Paris press-conference said the ceasefire was not peace but rested on the Houthis not bombing, shooting or carrying out any attacks across the country.

"A ceasefire is not peace, ultimately the parties will have to come back to the negotiating table," Kerry added.

Coalition air strikes have been pummeling Saada for more than a month since the start of a campaign against the rebels, who are allied with forces loyal to ousted President Ali Abdullah Saleh.

The Houthis overran the Yemeni capital, Sanaa, last September and are currently engaged in an offensive in southern Yemen and Aden - the south's main port city.

The offensive forced the legally installed President Abd-Rabbu Mansour Hadi to flee the country in late March and seek exile in Saudi Arabia.