SANAA, Yemen — 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.

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 media.

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The assault appeared to be a significant escalation in the U.S. and Yemeni campaign against al-Qaida in the Arabian Peninsula, the terror group’s powerful branch in the southern Arabian nation.

The United States has been striking al-Qaida positions in the country heavily with drone strokes the past two years, trying to cripple the group after it was driven out of several cities it took over in 2011.

But the group has proved highly resilient, spreading around the country and working from mountain areas.

In a show of the group’s boldness, a video recently posted on Islamic militant websites showed the group’s leader Nasser al-Wahishi meeting openly with a gathering of dozens of militants in the southern province of Abyan.

The base is in a remote mountain valley called Wadi al-Khayala in the rugged Mahfad region at the border between Abyan, and the neighboring provinces of Shabwa, and al-Bayda.

The first strikes came Sunday in an assault a high-level government security committee said was an attack on training grounds for the group.

Yemeni Interior Ministry said it lasted for several hours. Yemeni officials and tribal leaders said new strikes, believed to include U.S. drone hits, came Monday. Another airstrike Saturday in al-Bayda killed at least nine militants.

The ministry said in a statement Monday that the strikes the day before had killed at least 55 militants, including three prominent figures.

It identified the three as Mohammed Salem Abed Rabbo al-Mashibi, Fawaz Hussein al-Mahrak, Saleh Said Mahrak. It said identification of the dead was continuing, and that non-Yemeni Arab fighters were among those killed.

In Washington, a Pentagon spokesman said he could not comment on any specific actions.