Al-Qaeda claimed responsibility for two suicide bombings on May 25 against Shiite Zaidi rebels in northern Yemen, in a statement posted Friday on jihadist websites.
"Two lions of Islam carried out two operations in Saad and Al-Jawf provinces," mostly under rebel control, it said.
"Our battle against the crusaders and their lackeys will not prevent us from targeting the Rafidah (Shiites) because they attacked our religion and the honour of our prophet (Mohammed) and carried out criminal acts against Sunnis."
Thirteen people were killed as a suicide attacker drove an explosives-packed car into a school converted into a Zaidi rebel base in Al-Jawf, a tribal chief and witnesses said.
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Earlier the same day, another suicide attacker was killed as he tried to infiltrate a Zaidi-led rally also in Al-Jawf, according to a Zaidi rebel spokesman.
The Al-Qaeda statement said a second "martyr" on May 25 blew up an explosive belt in the middle of a Huthi marketplace in Saada, without mention of the foiled attack.
Ahead of the attacks, the Al-Qaeda-affiliated Partisans of Sharia (Islamic law) in Yemen threatened to launch attacks against Huthis in Al-Jawf, Hajja and Saada to seek revenge for attacks on Salafists in northern Yemen.
Dozens of people have been killed in sectarian clashes since last year between the rebels and Sunni Salafists trying to tighten their grip on the north, where government control has slackened since a political crisis in Sanaa.
Yemen's mountainous north is a stronghold of the Zaidis, also known as Huthis, who from 2004 fought six wars with central government forces before signing a truce in February 2010. The rebellion claimed thousands of lives.