King Salman Bin Abdul Aziz warned his country would strike with an “iron hand” against people who preyed on youth vulnerable to extremism, a day after suicide bombers struck three cities in an apparently coordinated campaign of attacks.

In a speech marking Eid Al Fitr, King Salman said a major challenge facing Saudi Arabia was preserving hope for youth who faced the risk of radicalisation.

“We will strike with an iron hand those who target the minds and thoughts... of our dear youth,” Salman, 80, said.

Four security officers were killed in Monday’s attacks that targeted US diplomats, Shiite worshippers and a security headquarters at a mosque in the holy city of Madinah. The attacks all seem to have been timed to coincide with the approach of the Eid holiday.

The UN human rights chief on Tuesday described the bombing outside the Prophet's Mosque in Madinah as “an attack on Islam itself” and many Muslims expressed shock that their second-holiest site had been targeted.

No group has claimed responsibility, but Daesh militants have carried out similar bombings in the kingdom in the past year.

Zeid Ra’ad Al Hussein, the UN High Commissioner for Human Rights and a member of the Jordanian royal family, delivered his remarks via a spokesman in Geneva.

“This is one of the holiest sites in Islam, and for such an attack to take place there, during Ramadan, can be considered a direct attack on Muslims all across the world,” he said. “It is an attack on the religion itself.”

Unprecedented

Militant attacks on Madinah are unprecedented.

“I apologise to everyone if I don’t congratulate you this Eid,” Khaled bin Saleh Al Shathri, a Saudi businessman, wrote on Twitter. “I am shocked by the deaths of five of my brothers and the wounding of four others as they guarded the holiest places.”

Saudi security officials say Daesh’s supporters inside the kingdom mainly act independently from the group in Iraq and Syria, its main areas of operations.

Salah Al Budair, the imam of the Prophet’s Mosque, warned young people about being lured by the “malignant” ideology of Daesh. “(The bomber) is an infidel who has sold himself to the enemies of his religion and his country,” he said.