Yemeni rebels have crossed a "red line" and will pay a high price for deadly bombardments of Saudi Arabia, the Saudi-led coalition said Thursday, hours after the kingdom proposed a humanitarian ceasefire.
Shells fired from Yemen this week killed several people in Saudi Arabia's border region, the first attacks on populated areas of the kingdom since a Saudi-led coalition began air strikes against the Shiite Huthi rebels on March 26.
"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.
"The safety of Saudi Arabia is a top priority for the coalition and the Saudi armed forces. It is a red line they crossed."
Assiri said an operation to respond "starts now" and will target "all the leaders of the organisation, the areas they gather, their leadership locations, and the planners. And it will not be a limited operation."
The Huthi stronghold of Saada, in northern Yemen, would be among the targets in the coalition's sights.
His threat came only hours after US Secretary of State John Kerry urged the Iran-backed rebels to accept Saudi Arabia's offer of a five-day renewable humanitarian ceasefire.
Saudi-led air strikes have failed to halt the Huthis in Yemen, and concern has been mounting over increasing civilian deaths.
- 'No bombing, no shooting' -
Saudi Foreign Minister Adel al-Jubeir announced "the kingdom's idea of a five-day ceasefire... to deliver aid" after talks with Kerry, who described it as a "major opportunity" to ease the crisis.
But Jubeir said it would only apply "if the Huthis and their allies commit to this and do not carry out acts of aggression".
A date for the ceasefire would "soon be set," he told a joint news conference with Kerry in Riyadh.
"No bombing, no shooting, no movement or repositioning of troops to achieve military advantage," Kerry said, adding the ceasefire could be renewable but rebels must meet those commitments.
The call coincided with a renewed UN plea for a humanitarian ceasefire in Yemen, where weeks of war have now killed more than 1,400 people and injured nearly 6,000 -- many of them civilians.
"I repeat my call for a humanitarian pause, to be observed by all parties, to allow civilians to escape conflict areas and access basic services, and to enable humanitarian agencies to provide life-saving assistance," said the UN's humanitarian coordinator for Yemen, Johannes van der Klaauw.
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- Qaeda says US killed commander -
Shiite militiamen and Sunni extremists have sought to exploit the power vacuum in Yemen since bloody year-long protests forced autocratic president Ali Abdullah Saleh to step down in early 2012.
Among them is Al-Qaeda in the Arabian Peninsula, which said Thursday in an online video that the US military killed one of its commanders, Nasser bin Ali al-Ansi, in an air strike last month, according to SITE Intelligence Group.
Ansi appeared in the AQAP video in which it claimed responsibility for a deadly January attack on French magazine Charlie Hebdo to avenge its cartoons of the Prophet Mohammed.
The United States, the only country that operates drones over Yemen, declined to confirm Ansi's death.
"I cannot give you a specific answer on a particular strike", US Defence Secretary Ashton Carter said. "I will say that on the general question of AQAP, we will continue to apply pressure."
During his talks in Yemen, America's top diplomat, Kerry, said there was no discussion about sending troops into Yemen, whose exiled government has appealed for action on the ground.
It came after Yemen wrote to the UN Security Council to "urge the international community to quickly intervene by land forces to save Yemen, especially Aden and Taez".
The Saudi-led coalition's strikes and weapons drops have been supporting fighters loyal to President Abedrabbo Mansour Hadi, who fled to Riyadh as the rebels advanced on his southern refuge in the port city of Aden where intense combat continues.
Hadi supporters are battling the Huthis, who have seized large parts of the country including Sanaa, the capital. The Huthis are allied with troops loyal to ex-president Saleh.
UN envoy Ismail Ould Cheikh Ahmed was also in Riyadh in a bid to relaunch peace talks.
On Wednesday, 38 people were reported to have died while trying to flee Aden by sea.
A health official accused the rebels of killing the civilians and wounding another 95 in the shelling of a fishing harbour and a barge.
The UN's van der Klaauw said he was "gravely concerned by reports coming out of Aden yesterday".
As well as the growing civilian death toll from the air campaign and fighting, deliveries of fuel, food and medicine have been severely crippled in the impoverished Arabian Peninsula nation.
Iran has dismissed as "utter lies" accusations it armed the Huthis but Jubeir said Tehran's support for the rebels included weapons.