Four people, including two security guards, have been killed in an explosion outside the Prophet's Mosque in the Saudi Arabia's Medina, Islam's second holiest city, sources tell Al Jazeera.
Photos on social media show smoke billowing from a fire outside the mosque where Prophet Muhammad is buried.
The cause of the explosion on Monday evening was not immediately known. Some reports suggested it was a suicide bombing, while others said a gas cylinder had blown up.
Qari Ziyaad Patel, 36, from South Africa, was at the mosque when he heard a blast just as the call to sunset prayers was ending.
Many at first thought it was the sound of traditional, celebratory cannon fire, but then he felt the ground shake.
"The vibrations were very strong," he told the AP news agency. "It sounded like a building imploded."
The blasts occurred just before the Maghreb (sunset) prayers when people were breaking their fast inside the mosque. 

Qatif explosions

Around the same time, two other explosions struck near a mosque in the eastern city of Qatif on the Gulf coast, residents said.
Witnesses said a suicide bomber blew himself up outside a Shia mosque without causing any other injuries.
They reported seeing body parts lying on the ground in the city's business district.
"Suicide bomber for sure. I can see the body" which was blasted to pieces, a resident told the AFP news agency.
Nasima al-Sada, another resident, said "one bomber blew himself up near the mosque".
A third witness told Reuters news agency that one explosion destroyed a car parked near the mosque, followed by another explosion just before 7pm local time.
"We are in the last 10 days of Ramadan and those places are crowded because of that for Maghreb [sunset] prayers," Khaled Batarfi, a Saudi Gazette columnist, told Al Jazeera.
There was no claim of responsibility for the attack.
 
Early on Monday morning, two security officers were injured as a suicide bomberblew himself up near the US consulate in the Red Sea port city of Jeddah.
Security officers became suspicious of a man near the car park of Dr Suleiman Faqeeh Hospital which is directly across from the US diplomatic mission. When they moved in to investigate, "he blew himself up with a suicide belt inside the hospital parking", the ministry said, adding that two security officers were lightly wounded.
In January, at least four people were killed in a suicide attack on a Shia mosque in the eastern al-Ahsa region.
In October, the Islamic State of Iraq and the Levant (ISIL) group claimed responsibility for a suicide bombing at a Shia mosque in Najran, in which at least one person was killed.
ISIL (also known as ISIS) had also claimed responsibility for an attack at a mosque inside a special forces headquarters in the city of Abha in August 2015. Fifteen people were killed in that attack.