The Saudi-led coalition on Saturday launched a "large-scale" assault on Yemen after missiles fired by Iran-backed Huthi rebels killed two people in the kingdom, the first such deaths in three years.
The Huthis warned Yemen's oil-rich northern neighbour of a "painful" response if the coalition does not stop its "aggression" against the conflict-riven country.
Yemen has been wracked by civil war since 2014 pitting the internationally recognised government supported by the Saudi-led military coalition against the Huthis who control much of the north.
Tens of thousands of people have since been killed, in what the UN has described as the world's worst humanitarian crisis.
The latest violence came overnight when two people -- one Saudi and the other Yemeni -- were killed in the projectile attack on Jazan, said Saudi's civil defence.
"A military projectile fell on a commercial store on the main street, resulting in two deaths," it said, adding six Saudis and a Bangladeshi national were wounded.
Images from the official Saudi Press Agency purportedly of the aftermath of the attack showed a large crater in the ground and destroyed vehicles.
- Huthis warn of 'painful' response -
The Saudi-led coalition said shortly afterwards that it was "preparing for a large-scale military operation".
It later launched an air strike in which "three civilians including a child and a woman were killed, and six others were wounded", Yemeni medics told AFP.
The coalition will hold a news conference on Sunday to address the latest developments, the Saudi authorities said, clarifying an earlier advisory that it would be on Saturday.
Huthi spokesman Yahya Saree warned Saudi Arabia of "painful operations as long as it continues its aggression and crimes".
In a statement on the Huthis' Telegram channel, he said the rebels had launched three ballistic missiles on Jazan, a southern region of the kingdom bordering Yemen.
The insurgents often launch missiles and drones into Saudi Arabia, targeting its airports and oil infrastructure.
The latest was the first in more than three years that has resulted in fatalities in the kingdom, which recorded its first death from a Huthi missile attack when a missile struck Riyadh in 2018.
It also comes as fighting between the two sides intensifies, with the coalition ramping air strikes on Sanaa.
- 'Deaths every day' -
Sunni-ruled Saudi Arabia and its ally the United States have long accused Iran of supplying the Huthis with sophisticated weapons, a charge the Shiite-dominated Islamic republic denies.
The US Navy said this week that it seized 1,400 AK-47 rifles and ammunition from a fishing boat it claimed was smuggling weapons from Iran to the Huthis, who are from Yemen's Zaidi Shiite minority.
"The stateless vessel was assessed to have originated in Iran and transited international waters along a route historically used to traffic weapons unlawfully to the Huthis in Yemen," it said.
On Thursday -- a day after the coalition targeted a Huthi military camp in Sanaa -- the military alliance said it shot down a bomb-laden drone near Abha airport in the south of the kingdom, causing debris to fall nearby but leaving no casualties.
And earlier this week, it targeted Sanaa airport, whose operations have largely ceased because of a Saudi-led blockade since August 2016, with exemptions for aid flights.
In his Christmas Day message, Pope Francis deplored the fact that "immense tragedies" in conflict-riddled Arab countries, including Yemen, were "being passed over in silence".
"Let us listen to the cry of children arising from Yemen, where an enormous tragedy, overlooked by everyone, has silently gone on for years, causing deaths every day," he said in the Vatican.
The World Food Programme said it has been "forced" to cut aid to Yemen due to lack of funds, and warned of a surge in hunger in the country.
The UN estimates Yemen's war will have claimed 377,000 lives by the end of the year through both direct and indirect impacts.
More than 80 percent of Yemen's population of about 30 million requires humanitarian assistance in what the UN says is world's worst humanitarian crisis.