Updated

Iraq tightened security Saturday at major Shiite shrines in response to two days of homicide attacks targeting worshippers — including one at a revered Baghdad tomb that was the single deadliest bombing in the country this year.

More than 150 people, many of them Iranian pilgrims, died in the attacks Thursday and Friday.

The increase in attacks in recent weeks has raised questions about the abilities of Iraq's security forces, and U.S. Secretary of State Hillary Rodham Clinton met with Iraqi leaders Saturday in Baghdad to assure continued American support even as it moves toward withdrawing its troops.

The number of dead in Friday's attack by two female homicide bombers at Baghdad's shrine of Imam Mousa al-Kazim reached 71, an Iraqi police official said Saturday.

A hospital official confirmed the toll, adding that 125 were also wounded in the bombing. Many were Iranian pilgrims on their way to pray at the popular mosque.

Friday's attack was the single most deadly this year. The death toll topped the 57 people killed Thursday by a homicide bomber at a restaurant north of Baghdad in Muqdadiyah, previously the deadliest attack since January. Another attack on Thursday struck Iraqis collecting humanitarian aid, killing 31.

Prime Minister Nouri al-Maliki has ordered a military task force to investigate Friday's attack. He also suspended the area commanders for failing to provide adequate security around the shrine.

The two blasts happened within minutes of each other at separate gates of the tomb shortly before the start of prayers as worshippers streamed into the mosque, police said.

Security has been tightened and checkpoints added around the shrine in the northern Baghdad neighborhood of Kazimiyah, a police official said.

The government also ordered additional security forces deployed to the southern holy cities of Najaf and Karbala, home to two other important Shiite shrines, another police official said.

All the officials spoke on condition of anonymity because they were not authorized to release the information.

Black banners bearing the names of the dead were hung Saturday at the site of the attacks, while candles and flowers were laid on the ground.

In Tikrit, a huge blast caused widespread damage to a wholesale market in Saddam Hussein's hometown, about 80 miles north of Baghdad.

Several warehouses holding food, appliances and clothes were lost in the fire, said a policeman, speaking on condition of anonymity because he was not authorized to speak to media. The cause was not immediately known.

A U.S. soldier was killed Saturday during an attack on a patrol in a northern Iraqi province that includes the disputed, oil-rich city of Kirkuk, according to a military statement.