Yemen’s Houthis said they launched missiles at Abha airport in southwestern Saudi Arabia on Monday in response to airstrikes on Sanaa airport, which they blamed on the kingdom.

The Saudi-led coalition in Yemen, which backs the country’s internationally recognized government, said its air defenses “dealt with” the missiles and no casualties were reported.

The Houthis, who control northwest Yemen and are backed by Iran, previously accused Saudi Arabia of “blatant aggression,” saying it had attacked the runway at Sana’a airport.

The attack was claimed by the Yemeni government, which said it wanted to prevent the landing of an Iranian plane.

It was the most significant escalation in the largely simmering conflict between the Houthis and Saudi Arabia since an informal truce took effect four years ago.

Yemen has been devastated by a civil war that began in 2014, when the Houthis overthrew the government in Sana’a, the capital. The conflict escalated in 2015, after the Saudi-led coalition of Arab states intervened in an attempt to restore the government.

The fighting has reportedly left more than 150,000 dead and triggered one of the world’s worst humanitarian crises, with more than 22 million people in need of some form of aid, according to the UN.

On Monday afternoon, images on social media showed plumes of smoke rising over the rooftops of Sana’a after the attacks on the city’s international airport.

The Houthis’ al-Masirah television said the “runs of departure and landing” were attacked.

The internationally recognized Yemeni government, based in the southern port of Aden, said its forces had carried out the attacks amid a dispute over the plane used by a Houthi delegation returning from Iran after the funeral of the late supreme leader Ayatollah Ali Khamenei.

“The Houthi terrorist militias, backed by the Iranian regime, prevented Yemeni national planes from landing at the airport in the capital Sanaa, while insisting on allowing an Iranian plane to violate Yemeni territory; consequently, the airport’s runway was attacked,” the Yemeni Defense Ministry said.

The Iranian plane had to divert and then landed in the Red Sea city of Hudaydah, about 150 kilometers (93 miles) to the southwest, according to the Houthis.

For more than a decade, planes entering Yemeni airspace have required authorization from the Saudi-led coalition, which says it is acting at the request of the Yemeni government.

Houthi military spokesman Yahya Saree accused Saudi Arabia of being behind the Sanaa attacks, which he said had ended “the de-escalation phase” of their conflict and would not go “unanswered or unpunished.”