LAGOS, Nigeria -- Nigerian troops Wednesday buried the bodies of hundreds of victims of an alleged military massacre of Shiite Muslims to hide the death toll, the Shiite Islamic Movement in Nigeria said.

Wednesday's allegation comes as human rights advocates and the United States called for an investigation following the army's raid on Nigerian Shiites in which hundreds of people were reportedly killed and Shiite leader Ibraheem Zakzaky suffered four bullet wounds.

The military said it acted after Shiites tried to assassinate Nigeria's army chief. Army spokesman Col. Sani Usman did not immediately respond to an email late Wednesday requesting comment on the burial charges.

Details of the weekend violence in Zaria have been slow to emerge because the three attacked areas of the northern town have been on lockdown with no one allowed to enter or leave.

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Shiite spokesman Ibrahim Musa said soldiers took the bodies from the mortuary of Ahmadu Bello University Teaching Hospital and buried them in mass graves on Wednesday. His statement did not explain how he got the information.

"The Nigerian army has desecrated our dead," Musa said. "We hereby demand the location of the mass burial, and the interrogation of those who ordered the operation."

Human rights groups say as many as 1,000 people may have been killed.

"The United States calls on the government of Nigeria to quickly, credibly, and transparently investigate these events in Zaria and hold to account any individuals found to have committed crimes," the U.S. Embassy said in a statement.

"It is essential that all sides refrain from actions that further destabilize the situation," U.S. Ambassador James Entwhistle added.

Amnesty International said in a statement late Tuesday that the shooting of members of the Shiite group in Zaria "must be urgently investigated ... and anyone found responsible for unlawful killings must be brought to justice."

"Whilst the final death toll is unclear, there is no doubt that there has been a substantial loss of life at the hands of the military," said M.K. Ibrahim, director of Amnesty International, Nigeria.

But the Shiite movement, which has millions of followers in Nigeria, rejected the investigative panel it said has been set up following a visit to Zaria on Tuesday by Interior Minister Abdulrahman Dambazau.

Musa said it has no confidence in a panel led by the area police commander because he is junior in rank to the alleged perpetrators of the killings.

It said Dambazau, a retired general, did not even bother to visit wounded victims in the hospital.

The bloodshed was yet another blow to Africa's most populous nation, already beset by a six-year-old insurgency waged by Boko Haram, a violent Islamic group which is at odds with the Shiites and others who oppose its extremist vision.

In addition to Wednesday's alleged mass burial, Musa said soldiers on Monday carried away about 200 bodies from around the home of Zakzaky, and did not deposit them at the hospital mortuary.

The military has said Zakzaky is in its "safe custody."

The army said troops attacked sites in Zaria after 500 Shiites blocked the convoy of Nigeria's army chief, and tried to kill him on Saturday. A report from the military police said some Shiites were crawling through tall grass toward Gen. Tukur Buratai's vehicle "with the intent to attack the vehicle with (a) petrol bomb" while others "suddenly resorted to firing gunshots from the direction of the mosque."

In a statement Monday, the army said both the military and Shiites lost lives but a toll still is being compiled.

The National Human Rights Commission of Nigeria said it set up a special investigations panel on Tuesday following complaints filed by concerned citizens and from the military, which has asked the commission to investigate the alleged assassination attempt on the army chief.

The commission's chairman, Chidi Odinkalu, called the army attacks "a massacre." He said one of Zakzaky's wives was killed. Two of Zakzaky's sons also were killed and one was wounded, according to Musa.

Iran, seen as the guardian of the Shiite Muslim faith, has condemned the killings.

Iranian President Hassan Rouhani called Nigerian President Muhammadu Buhari. Iran state TV said Rouhani told Buhari Tuesday that he expects the Nigerian government to compensate families of victims.

Hundreds of Shiites protested in front of the Nigerian embassies in the Iranian and Indian capitals on Tuesday.

Nigeria's Shiites, a movement of millions started 37 years ago by Zakzaky, who dresses in the robes and turban of an Iranian ayatollah, often have clashed with police and other security forces over their unlawful blocking of major roads to hold religious processions.

Nigeria's military is infamous for its excesses. In 2009, Nigerian armed forces attacked Boko Haram's headquarters and killed about 700 people, including its leader. Still, Boko Haram re-emerged as a much more violent entity.

On Wednesday, Sen. Shehu Sani, a human rights activist, said "The Zaria killings simply affirmed the fact that as a nation we have not learnt our lessons from our painful and hard experience.

"The action of the military is untenable, intolerable and unacceptable ... and a step toward tyranny."

The Shiites two weeks ago suffered a suicide bombing in a procession that killed 22 people. Boko Haram, a Salafist group, claimed responsibility for the attack and threatened to "wipe out" the Shiites.