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There were three suicide bomb attacks in Saudi Arabia tonight - the most serious in the city of Medina, Islam's holiest site after Mecca.

Although nobody is yet to claim responsibility, the attacks appear to be part of a coordinated campaign as Saudis prepared to break their fast on the penultimate day of the holy month of Ramadan.

The explosions targeting U.S. diplomats, Shi'ite worshippers and a security headquarters at the mosque follow days of mass killings claimed by Islamic State terrorists.

ISIS has committed atrocities and mass killings in Turkey, Bangladesh and Iraq in the last week.

The attacks today all seem to have been timed to coincide with the approach of Eid al-Fitr, the holiday that celebrates the end of the fast.

At least two people have reportedly been killed in an explosion outside the Prophet's Mosque - where the Prophet Muhammad is buried - sources told Al Jazeera.

There are also reports of blasts - but failed attacks - in Qatif and Jeddah.

(Image: Reuters)

Photos on social media taken in Medina show smoke billowing from a fire outside the mosque - and security guards lying nearby.

Other footage showed ambulances and other emergency vehicles rushing to the scene.

Saudi-owned al-Arabiya television said an initial death toll from the Medina blast included three suicide bombers and two security forces officers.

Images obtained from Reuters showed flames and smoke rising from the vicinity of the mosque.

Al-Arabiya said an initial death toll from the Medina blast included three suicide bombers and two security forces officers.

The cause of the explosion was not immediately known - but several reports blame a suicide bomber.

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The local al-Arabiya TV said that the suicide bomber detonated his device as security officers were breaking their Ramadan fast.

(Image: REUTERS/Handout)

Another explosion has rocked the eastern city of Qatif in a blast which appeared to target a Shia mosque.

At least one and possibly two explosions struck near the mosque.

There are no details of the incident or casualties yet - although some claim only the attacker died.

Witnesses described body parts, apparently of a suicide bomber, in the aftermath.

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And another suicide bomber also died after detonating a device in the city of Jeddah - the Saudi kingdom's second city - near the US consulate.

(Image: REUTERS/Handout)

Two security officers were slightly injured as they tackled the man - but nobody else was reportedly hurt in that incident.

It is not immediately clear who is behind any or all of the attacks.