The al-Askari shrine's dome was destroyed in February 2006 in a bombing blamed on al Qaida-linked militants. The mosque compound and minarets remained intact, and have been guarded since then by Iraqi police and commandos.
The US military headquarters in the area had no immediate information about today's attack. "We're only hearing initial reports ourselves, and we're looking into it," said Capt. Jennifer Nihill, a spokeswoman for Task Force Lightning.
The bombing could possibly unleash another wave of sectarian killings similar to the one that swept Baghdad and other areas of Iraq in the months that followed the destruction of the shrine's dome.
The sectarian killings have declined since February, at the start of a major US-Iraqi security push to pacify Baghdad.
Since the shrine's famous golden dome was destroyed in the bombing on February 22, 2006, the mosque has remained closed, the dome a mound of rubble.
The al-Askari mosque contains the tombs of the 10th and 11th imams - Ali al-Hadi, who died in 868, and his son Hassan al-Askari, who died in 874. Both are descendants of the Prophet Mohammed, and Shiites consider them to be among his successors.
The shrine also is near the place where the 12th imam, Mohammed al-Mahdi, disappeared. Al-Mahdi, known as the "hidden imam," was the son and grandson of the two imams buried in the Askariya shrine. Shiites believe he will return to Earth restore justice to humanity.Read More
The explosions were heard in the vicinity of the Askariya mosque at about 9:00am (0500 GMT) on Wednesday. There were no immediate reports of casualties.
"The explosion targeted the two golden minarets. They have been damaged ... This is a criminal act which aims at creating sectarian strife," Saleh al-Haidari, the head of the Shia endowment in Iraq , said.
The two minarets toppled on Wednesday even as security forces were guarding the holy site.
Nuri al-Maliki, Iraq' s prime minister, has blamed the attack on al-Qaeda and supporters of Saddam Hussein , Iraq ’s former president.
"How did the terrorists manage to go inside the shrine, plant the explosives and then blow it up, unless they have some people in the forces there helping them?"
Abdul Mahdi al-Mutiri, senior spokesman for the group loyal to Muqtada al-Sadr The Askariya mosque was already severely damaged after a February 2006 attack blamed on al-Qaeda fighters.
"I was near the shrine when I heard big explosions that sent a thick cloud of dust in the sky covering the entire area," said a witness.Read More
The two minarets of the al-Askari shrine in Iraq, one of the holiest sites in Shia Islam, have been destroyed by two explosions.
According to witnesses the minarets collapsed completely after being hit by bomb blasts at around 0900 (0500 GMT).
The shrine houses one of two tombs in Samarra for revered Shia imams.
The 2006 bombing of the shrine's dome is widely believed to have set off a continuing spiral of sectarian violence in which many thousands have died.
The BBC's Jim Muir in Baghdad says there are obvious fears this latest attack might give it yet further impetus.Read More