A branch of the self-declared Islamic State has claimed responsibility for the suicide bombing of a Shiite mosque in eastern Saudi Arabia that has killed at least 19 people, a move that could represent a significant escalation of the extremist group's operations in the kingdom.

NPR's Leila Fadel reports from Cairo that the online statement from ISIS "named and praised the Saudi suicide bomber who detonated himself amongst a congregation of Shiite Saudis praying in a mosque in the village of al Qudaih in Qatif province."

She says it is the first time a Saudi branch of ISIS known as Najd Province has claimed responsibility for an attack inside the kingdom. Leila says Saudi Shiites are concerned that the attack represents a backlash for Saudi Arabia's military campaign against rebel (Shiite) Houthis in Yemen.

"In the statement, ISIS called Shiite Muslims impure and vowed that dark days are ahead for them. It also said attacks like this one won't stop until they drive all Shiites from the Arab peninsula," Leila reports.

According to The New York Times: "During Saudi Arabia's two-month air campaign against the Houthi movement in Yemen, which practices a form of Shiite Islam and receives backing from Saudi Arabia's regional rival, Iran, imams at Sunni mosques and commentators in Saudi Arabia have frequently rallied the public around the war, in part by repeatedly denouncing Shiites as dangerous infidels."

Al-Jazeera offers a bit of background:

"Saudi Arabia's Shia population is mostly based in two oasis districts of the Eastern Province — Qatif on the Gulf coast, and al-Ahsa, southwest of the provincial capital al-Khobar.

"Qatif and al-Ahsa have historically been the focal point of anti-government demonstrations.

"The kingdom's Shia community accounts for between 10 to 15 percent of the total population. They say they face discrimination in seeking educational opportunities or government employment and that they are referred to disparagingly in text books and by some Sunni officials and state-funded clerics."

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