Yemen’s Houthi rebels said Friday they launched a drone attack on a military airbase and an airport in Saudi Arabia, stepping up their offensive on the kingdom’s energy and security installations. The attack, using bomb-laden drones, comes a day after they hit a Saudi Aramco fuel depot in Jeddah with a Quds-2 cruise missile, roiling oil markets.

It was not clear how much damage the strikes had caused. Houthi attacks on Saudi Arabia rarely claim lives or cause extensive damage but their frequency has increased, creating unease in the Gulf, a region key to global oil production and transit.

The Houthis have been fighting Yemen’s United Nations-recognized government since 2014 and have taken over the capital Sanaa. A Saudi-led coalition intervened the following year on the side of the government.