The Islamic State group has claimed responsibility for a mosque bombing in Saudi Arabia, CNN reported Friday. The attack occurred in the eastern part of the country, killing at least 21 people.

The bombing took place during midday prayers at the Imam Ali mosque in the village of al-Qadeeh. This was the first time ISIS has claimed to have performed an attack inside the Saudi kingdom. However, the claim has not been independently confirmed yet. It is believed that the attack is in retaliation for Saudi Arabia’s involvement in the Yemen conflict.

Even though this seems to be the first ISIS attack in Saudi Arabia, the militant group has earlier threatened the kingdom. ISIS identified Abu Amer al-Najdi as the suicide bomber. The extremist group credited "the soldiers of the Caliphate" for the attack.

According to witnesses, there was a huge blast at the mosque in the Qatif governorate. A local doctor said that at least 70 people had been injured in the attack. Television pictures showed debris and shattered glass inside the Shiite mosque. Dozens of people were reported to be praying when the bombing happened.

According to the Saudi interior ministry, a suicide belt was detonated inside the mosque. "Security authorities will spare no effort in the pursuit of all those involved in this terrorist crime," a spokesman said.

The New York Times reported that Saudi Arabia’s air campaign in Yemen went on for two months. The campaign was against the Houthi movement in the second-largest country in the Arabian Peninsula.

Saudi Arabia’s clerical establishment is often criticized by the Shiite minority in the country. The community, consisting of around 15 percent of the total population, accuses the Sunni majority of discrimination and insult.

ISIS shares views with hard-line Sunni leaders who do not consider Shiites as “real Muslims” and often indulge in hate speech. BBC security correspondent Frank Gardner says that it does not come as a surprise that ISIS is behind the calculated attack against “peaceful worshippers.”

ISIS’ Saudi branch was established only in November 2014. In an online statement, the militant organization announced that it had carried out a similar attack in Yemen that killed around a dozen people Friday.