Saudi authorities have confirmed that operations by Yemen’s armed forces against two of the kingdom’s international airports killed three people and wounded at least 36 others, underscoring the growing cost of Riyadh’s years-long aggression against Yemen.

The kingdom’s General Authority of Civil Aviation said on Thursday morning facilities at Abha International Airport and King Khalid International Airport in Riyadh were struck on Tuesday and Wednesday.

Two female residents, one Moroccan and one Algerian, were killed at Abha and 28 others were wounded. A Sudanese resident was killed at King Khalid Airport, where eight people were injured, including five Saudi citizens.

Yemeni military spokesman Yahya Saree said the armed forces had targeted King Khalid International Airport with a ballistic missile, disrupting air traffic in the Saudi capital, and had also struck Abha airport along with military sites in the southwest of the kingdom.

Saudi-led coalition spokesman Turki al-Malki later claimed retaliatory raids against more than 80 positions in Saada, Hodeidah, Jawf and Marib, vowing that the operations “will not go unpunished.”

The strikes prompted a security alert from the US Mission in Saudi Arabia. In an advisory issued Wednesday, the mission told American citizens to exercise heightened vigilance, citing public threats by Yemeni forces against airports and airspace, the risk of further escalation, and the possibility of flight cancellations, airspace closures and travel disruptions inside the kingdom.

The latest exchanges come as fighting between the Yemeni armed forces and Saudi-backed militias has already displaced more than 200,000 people.

Observers note that the current pressure on Saudi territory is the direct consequence of a war Riyadh launched in 2015.

After intervening militarily, Saudi Arabia imposed a land, air and sea blockade on Yemen; the United States joined that blockade in October 2016 before later claiming to have stepped back.

Sana’a’s decision to break the siege was met with Saudi bombing of the capital’s airport and a decisive response from Yemeni forces.

In subsequent operations, they pushed back Saudi-backed militias, captured the historic port of al-Mukha and the coastal area of Kahboub, and retook a number of strategic islands. Those gains have consolidated Yemeni control over approaches to the Bab al-Mandeb Strait, one of the world’s most important shipping lanes, and demonstrated that the era of one-sided Saudi attacks on Yemeni soil has ended.