Sanaa, Yemen - A Houthi official on Monday warned the US it would receive a "painful response" if it entered the war in Yemen, where Saudi Arabia has reportedly requested Washington's help in dealing with the group as it weathers repeated attacks.
Yemen's civil war – in which a Saudi-led military coalition supports the government against the Iran-backed Houthis – reignited in July, months into the wider regional conflict that has roiled energy markets and plunged the Middle East into crisis.
The Houthis quickly began targeting Saudi oil facilities, tankers and transport hubs, including with a string of lethal strikes on Riyadh's airport, rocking the world's top oil exporter.
Following an attack Saturday at the airport that killed 13 people, US President Donald Trump said he was considering joining Saudi strikes against the group, after having reportedly rebuffed previous requests for help from the kingdom.
"If the US becomes involved in the aggression against Yemen, we will not deliberate over the decision to respond, rather, there will be a swift and painful response targeting the US and all its interests in the region," Houthi political bureau member Mohammed Al Bukhaiti told AFP in an interview.
He also stated "all employees and workers at Saudi oil facilities and airports to stay away from these sites, as they have become legitimate targets."
Saturday's strike on King Khalid International Airport was the deadliest attack on Saudi soil in more than a decade, and also left hundreds wounded.
Trump said the attack was "not acceptable" and that he would be speaking to the kingdom's de facto leader, Crown Prince Mohammed bin Salman, telling reporters the US was "going to look at" the possibility of joining Saudi strikes.
Four people died in earlier strikes on Wednesday and Thursday, and another was killed on Sunday. Operations remained suspended at the sprawling airport on Riyadh's outskirts on Monday.
Bukhaiti said the purpose of the campaign against Saudi targets was to send a message to the crown prince "that he can no longer wage aggression against Yemen from a safe position."
Houthi attacks call for "serious response" by Saudi Arabia as flights get suspended
Saudi energy minister Prince Abdulaziz bin Salman told an energy conference in Riyadh on Monday that the recent events had "tested our infrastructure, our institutions, our people, and our resilience."
"The events of recent weeks are serious, our response must be equally serious."
CCTV footage of Saturday's attack released by Saudi authorities showed Muslim pilgrims in religious attire waiting in departure lounges before a sudden blast tore through the building and brought down flaming debris.
The atmosphere in Riyadh remained tense on Monday, as state broadcaster Al Ekhbariya said the military coalition was responding to drone and missile threats, and Australia's embassy warned its citizens in the city to shelter in place.
The US embassy in Saudi Arabia said US government employees must obtain its authorisation before using any airport in the kingdom, and urged US citizens to "take extreme caution" if they chose to do so.
The mission also said "American citizens have been detained for taking photos of recent events" in Saudi Arabia, without specifying how many or whether they had been released.
The UAE's national carrier Etihad Airways announced on Monday that it was cancelling all flights to and from Riyadh until next week due to "regional disruption," and also suspended flights to Dammam and Jeddah.
Bahrain's national carrier Gulf Air said it too was cancelling all Riyadh flights until Saturday.
US operator Delta announced it would delay the launch of direct flights from Atlanta to Riyadh – scheduled for October 23 – as US air safety regulators warned of continued safety risks.
Despite vast spending on military hardware, the Houthi attacks on oil, military and transport facilities across the large, mostly desert country are stretching Saudi defences.
Thomas Juneau, security analyst at Ottawa University, said that despite some success, "Saudi Arabia is so large that it's simply impossible for them to consistently defend themselves."