SANAA (AFP) - Saudi Arabia on Friday announced a humanitarian ceasefire in Yemen starting May 12, even as it stepped up retaliatory air strikes on Shi'ite Huthi rebels in the north of the country.

"We have made a decision that the ceasefire will begin this Tuesday, May 12, at 11pm and will last for five days subject to renewal if it works out," Saudi Foreign Minister Adel al-Jubeir said at a meeting of Gulf ministers in Paris.

Warplanes from a Saudi-led coalition have focused their fire since Thursday on rebels in their northern stronghold after Riyadh vowed "harsh" punishment for deadly cross-border bombardments.

On the humanitarian front, the UN children's agency warned that restrictions on delivering food and fuel are hampering aid efforts and threaten the lives of tens of thousands of Yemeni children.

"The ceasefire will end should Huthis or their allies not live up to the agreement - this is a chance for the Huthis to show that they care about their people and they care about the Yemen people," Jubeir said at a joint news conference with US Secretary of State John Kerry.

Raids on Thursday targeted rebel control centres, a communications complex, a landmine factory and other rebel positions across Saada province bordering Saudi Arabia, state media in Riyadh said.

Witnesses in Saada said coalition jets dropped leaflets urging residents to leave and an AFP correspondent in Sanaa reported scores of families arriving in the capital.

'ALL OF SAADA MILITARY TARGET'

The coalition warned that "all of Saada will be a military target to coalition strikes after 7pm (11pm)" Friday, Saudi media said.

Saudi Arabia, which has led six weeks of air strikes on Yemen in support of exiled President Abedrabbo Mansour Hadi, has warned that the Iran-backed Huthi rebels crossed a "red line" by shelling populated border areas in the kingdom.

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

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

Riyadh has repeatedly accused Iran of arming and funding the Huthis, charges Teheran denied again on Friday, with a foreign ministry spokeswoman dismissing "efforts to put the blame on others".

Assiri's warning came just hours after Kerry urged the rebels to accept Riyadh's offer of a five-day renewable humanitarian ceasefire.

Air strikes have failed to halt the Huthis and allied fighters loyal to former president Ali Abdullah Saleh, amid mounting concern over civilian casualties.

The United Nations has renewed its plea for a ceasefire, where weeks of war have now killed more than 1,400 people and wounded nearly 6,000, many of them civilians.

Unicef warned that fuel in Yemen may run out in less than a week, complaining that humanitarian access is being blocked by many parties to the conflict.

"If restrictions on the commercial imports of food and fuel continue, then it will kill more children than bullets and bombs in the coming months," said Unicef spokesman Christophe Boulierac.

UN envoy Ismail Ould Cheikh Ahmed met Hadi in Riyadh on Friday as he tries to relaunch stalled peace talks, Hadi's office said.

WOMEN, CHILDREN KILLED

Yemeni state news agency Saba, which the rebels control, said Friday that 13 people including women and children were killed when a village in the northern Hajja province was bombed.

Coalition warplanes hit Huthi positions in the city of Amran, north of Sanaa, witnesses said.

Overnight they targeted rebels in the southern port of Aden, the scene of clashes with Hadi allies.

A military official close to the rebels said 12 Huthi and Saleh loyalists were killed.

Aden's health authority chief Al-Khader Laswar said three civilians and three southern fighters were killed in clashes and 36 civilians were wounded.

Coalition aircraft also hit the eastern outskirts of rebel-held Ataq, the Shabwa provincial capital, as tribal fighters attacked on the ground, a military official close to the rebels said.

In the southern town of Loder, a roadside bomb killed 12 Huthis, local commander Ali Issa said, adding that two of his men died in clashes.

AFP could not independently verify death tolls.

The rebels are accused of receiving support from Tehran where thousands of people demonstrated Friday to denounce the air strikes.

Protesters paraded an effigy of Saudi King Salman holding an American flag in one hand and an Israeli one in the other.

Thousands of Huthi supporters also demonstrated in Sanaa on Friday, an AFP correspondent reported.