Yemen’s Iran-aligned Houthi group said on Tuesday it had attacked Saudi Arabia’s Ras Tanura oil refinery, the kingdom’s largest, located on the Gulf coast west of the Strait of Hormuz.

The group’s military spokesperson, Yahya Saree, said in a statement that the ballistic missile attack had caused a fire at the site. Saudi authorities have not confirmed the attack.

The Houthis have stepped up attacks on Saudi Arabia since September, opening another volatile front in the wider US-Israeli war on Iran.

Nearly all of Saudi Arabia’s oil exports pass through two main ports: Ras Tanura in the Gulf and Yanbu on the Red Sea. The Houthis have attacked both ports in the past.

Bloomberg News reported on Tuesday, citing a person familiar with the matter, that Saudi Arabia had contained a fire near the oil terminal at Ras Tanura in the early hours of the day.