Yemen’s Houthi rebels said they launched a bomb-laden drone into Saudi Arabia targeting an airport with a military base – an attack acknowledged by the kingdom.

It was not clear if there were any injuries or what the extent of damage was.

The Houthis’ Almasirah satellite news channel said early on Tuesday the attack hit the airport in Najran with a Qasef-2K drone, striking an “arms depot”.

Najran, 840km southwest of Riyadh, lies on the Saudi-Yemen border and has repeatedly been targeted by the Houthis.

A statement earlier on the state-run Saudi Press Agency quoted Saudi-led coalition spokesman Colonel Turki al-Maliki as saying the Houthis “had tried to target” a civilian site in Najran, without elaborating.

Al-Maliki warned there would be a “strong deterrent” to such attacks and described the Houthis as the “terrorist militias of Iran”.

Civilian airports throughout the Middle East often host military bases.

Americans in Najran?

The New York Times last year reported that US intelligence analysts were based in Najran, assisting the Saudis and a US Army Green Berets deployment on the border.

The Pentagon and the US military’s Central Command did not immediately respond to requests for comment.

The Yemen conflict has killed tens of thousands of people since a Saudi-led military coalition intervened with a devastating aerial bombardment campaign in support of the beleaguered government in March 2015, after the Houthis removed President Abd-Rabbu Mansour Hadi and forced him out of Sanaa, the capital.

The fighting has triggered what the UN describes as the world’s worst humanitarian crisis with 3.3 million people  displaced and 24.1 million – more than two-thirds of the population – in need of aid.

Last week, the Houthis launched a coordinated drone attack on a Saudi oil pipeline amid heightened tensions between Iran and the United States.

Earlier this month, officials in the United Arab Emirates (UAE) alleged that four oil tankers were sabotaged and US diplomats relayed a warning that commercial airlines could be misidentified by Iran and attacked, something dismissed by Tehran.

‘Economic war’

The attack on the Saudi city of Najran came as Iran announced it has quadrupled its uranium-enrichment production capacity amid tensions with the US, underscored by an exchange of threats and taunts on Monday on Twitter between President Donald Trump and Iran’s foreign minister.

Iranian nuclear officials made a point to stress the uranium would be enriched only to the 3.67 percent limit set under the 2015 nuclear deal with world powers, making it usable for a power plant – but far below what is needed for an atomic weapon.

But by increasing production, Iran soon will exceed the stockpile limitations set by the accord.

Tehran has set a July 7 deadline for Europe to set new terms for the deal, or it will enrich closer to weapons-grade levels in a Middle East already on edge.

The US has deployed bombers and an aircraft carrier to the Gulf over still-unspecified threats from Iran.