Warplanes from a Saudi-led coalition have struck Yemeni rebels in their northern stronghold as Riyadh vowed "harsh" punishment for deadly cross-border bombardments, shortly after proposing a humanitarian ceasefire.
The UN children's agency warned Friday that restrictions on delivering food and fuel are hampering aid efforts and threaten the lives of tens of thousands of Yemeni children.
Thursday's raids targeted control centres, a communications complex, a landmine factory and other rebel positions across Yemen's northern Saada province bordering Saudi Arabia, state media in Riyadh said Friday.
Witnesses in Saada said coalition jets dropped leaflets urging residents to leave and an AFP correspondent in Sanaa reported scores of families arriving on Friday.
The coalition warned that "all of Saada will be a military target to coalition strikes after 7:00 pm (1600 GMT)" Friday, Saudi media said.
Saudi Arabia, which has led six weeks of air strikes on Yemen in support of exiled President Abedrabbo Mansour Hadi, warned Thursday that Iran-backed Shiite Huthi rebels had 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.
- Children at risk -
"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 Tehran denied again on Friday, with a foreign ministry spokeswoman dismissing "efforts to put the blame on others".
Assiri's warning came just hours after US Secretary of State John 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 Friday as he tries to relaunch stalled peace talks, Hadi's office said.
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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.
- Iranians denounce raids -
Overnight they targeted rebels in the southern port of Aden, where clashes with Hadi allies continue.
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.
Shiite militiamen and Sunni extremists have sought to exploit the power vacuum since bloody year-long protests forced out longtime autocrat Saleh in early 2012.
Among those jostling for influence is Al-Qaeda in the Arabian Peninsula, which said Thursday that the US military had killed one of its commanders, Nasser bin Ali al-Ansi, in an air strike last month, SITE Intelligence Group reported.
Ansi appeared in an AQAP video claiming a deadly January attack on French magazine Charlie Hebdo to avenge its publication of cartoons of the Prophet Mohammed.