Explosions shook the cities of ancient Arabian cities of Medina and Qatif on Monday only hours after a thwarted attack on a third Saudi city.
A suicide bomber detonated a device near the security headquarters of the Prophet's Mosque in Medina, the second-holiest site in Islam, Saudi-owned al-Arabiya television reported.
The blast followed two others in Qatif, a city on the Persian Gulf that is home to many of the kingdom's Shi'ite minority. A source from the Ministry of Interior confirmed to NBC News the attacks were suicide bombings.
A witness to one of Qatif blasts said one explosion destroyed a car parked near a mosque, followed by another explosion just before 7 p.m. local time.
Photographs purporting to be of the incident published on Twitter showed a severed leg and crowds gathered outside a mosque at dusk. Some posts said the explosion was caused by a suicide attacker setting off a bomb.
The incidents happened just hours after a suicide bomber was killed and two people were wounded in a blast near the U.S. consulate in the kingdom's second city of Jeddah early Monday.
It was the first bombing in years to attempt to target foreigners in the kingdom. There was no immedaite claim of responsibility for the Jeddah attack.
ISIS has carried out a series of bombing and shooting attacks in Saudi Arabia since mid-2014 that have killed scores of people, mostly members of the Shi'ite Muslim minority and security services.