Yemen’s Iran-aligned Houthi group said it launched a drone attack on Jizan airport in southwestern Saudi Arabia early on Wednesday, part of an escalation of cross-border assaults in the 4-year-old conflict.

Axar.az reports citing Reuters that Houthi military spokesman Yahya Saria said the attack disrupted operations at the airport. There was no immediate comment from Saudi Arabia.

The Houthis, who are fighting a Saudi-led coalition in Yemen and control the capital Sanaa, have in the past few months stepped up their attacks against targets in Saudi Arabia. In response, the coalition has targeted military sites belonging to the group, especially around Sanaa.

On Tuesday, the Saudi-led coalition said it intercepted and downed three Houthi drones launched toward the southwestern Saudi cities of Jizan and Abha.

Saria said that Tuesday’s drone attack had targeted the King Khalid air base near the southwestern Saudi city of Khamis Mushait.

The escalation in violence threatens a U.N.-sponsored deal for a ceasefire and troop withdrawal from the flashpoint coastal city of Hodeidah, which became the focus of the war last year when the coalition tried to seize its port, the Houthis’ main supply line and a lifeline for millions of Yemenis.

On Monday the United Nations said the warring parties had agreed new measures to enforce the ceasefire and facilitate a troop pullback from Hodeidah.

The Western-backed Sunni Muslim coalition led by Saudi Arabia and the UAE intervened in Yemen in 2015 to try to restore the internationally recognized Yemeni government that was ousted from power in Sanaa by the Houthis in late 2014.

The war has killed tens of thousands of people and left millions on the brink of famine.