SANAA, Yemen (AP) — Yemeni forces, reportedly backed by U.S. drone strikes, hit al-Qaida militants for a second straight day Monday in what Yemen officials said was an assault on a major base of the terror group hidden in the remote southern mountains. The government said 55 militants were killed so far.

The sprawling base was a rare instance of a permanent infrastructure set up by al-Qaida’s branch in the country, Yemeni security officials said. Built over the past months, it includes a training ground, storehouses for weapons and food and vehicles used by the group to launch attacks, they said, speaking on condition of anonymity because they were not authorized to release details to the press.