SANAA, Yemen — Doctors Without Borders says an airstrike has hit a hospital it was operating in northern Yemen, killing and wounding an unknown number of people.

In social media posts, it says the Monday strike hit near the Houthi rebel stronghold of Saada, in the country's north. Yemeni security and medical officials, who spoke on condition of anonymity because they weren't authorized to brief the media, say the strike killed and wounded some 20 of the hospital's staff and patients.

The conflict in Yemen pits an internationally-recognized government backed by a Saudi-led coalition against the Shiite rebels, who captured the capital, Sanaa, in September 2014. The Saudi-led coalition has been carrying out airstrikes in Yemen since March 2015.