No group has yet claimed responsibility, but Saudi Arabia's Interior Ministry says all evidence points towards the DAESH terrorist group.
The king of Saudi Arabia warned his country would strike with an "iron hand" against people who preyed on youth vulnerable to religious extremism, a day after suicide bombers struck three cities in an apparently coordinated campaign of attacks.
In a speech marking Eid al Fitr, the holiday that celebrates the end of the Islamic holy month of Ramadan, King Salman (80) 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 said.
Four security officers were killed in Monday's attacks that targeted US diplomats, Shiite Muslim worshippers and a security headquarters at a mosque in the holy city of Medina.
The attacks all seem to have been timed to coincide with the approach of the Islamic Eid holiday.
The UN human rights chief on Tuesday described the bombing outside the Prophet Muhammed's Mosque in Medina 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, a terrorist organisation, have carried out similar bombings in the US-allied, Sunni Muslim-ruled kingdom in the past year, targeting minority Shiites and Saudi security forces.
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."
Attack Unnerves Saudis
Militant attacks on Medina are unprecedented.
The city is home to the second most sacred site in Islam, a mosque built in the 7th century by the Prophet Muhammed, the founder of Islam, which also houses his tomb.
Attacks on Mecca, the holiest place in Islam, have been extremely rare.
The Al Saud ruling family considers itself the protectors of both sites.
DAESH says the Saudi rulers are apostates and has declared its intention to topple them.
Iran, the region's major Shiite power, also condemned the attacks.
Saudi Arabia's crown prince and anti-terror tsar, Mohammed bin Nayef bin Abdul-Aziz, sought on Tuesday to reassure Saudis of the country's security.
"The security of the homeland is good, it is at its highest levels and thanks be to God it gets stronger every day," the state news agency SPA quoted him as saying during a visit to some of the wounded in the Jeddah attack.
Prince Mohammed has been credited for ending a bombing campaign by al Qaeda in Saudi Arabia between 2003 and 2006.
http://www.trtworld.com/mea/4-killed-by-suicide-bombings-in-saudi-arabia... happened days before the end of Ramadan, when Muslims fast from dawn until dusk.
Saudi security officials say Islamic State'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.