The U.S. military has accused Iranian-backed Shi'ite extremists of a bombing in an pet market in Baghdad Friday that killed at least 13 people and wounded nearly 60 others.

Rear Admiral Gregory Smith says Iraqi and coalition forces, conducting overnight raids, detained four members of a militia extremist group believed responsible for the bombing.

He says confessions and other intelligence led the military to believe the bombing was carried out by an Iranian-backed "special groups" cell operating in Baghdad.

"Special groups" is the U.S. military term for Shi'ite extremists who have broken away from radical cleric Moqtada al-Sadr. He has called on his fighters to lay down their weapons.

Admiral Smith says there was no evidence the Iranian government ordered the attack. He says the militants wanted it to look like the work of al-Qaida.

The bomb was packed with ball-bearings to maximize casualties.

Separately, the U.S. military says Iraqi forces detained four suspected militants, including two alleged extremist leaders, in separate overnight operations in the capital and Samarra.

The military says the insurgents are reported to be responsible for many attacks against Iraqi and coalition soldiers, as well as the killings of Iraqi citizens in the Beida area of Baghdad.

And the U.S. military says Iraqi police captured two suspected militants in an overnight operation to disrupt a terrorist network in the northern town of Samarra.

Some information for this report was provided by AFP, AP and Reuters.