(Adds PM Maliki in Mosul)

By Michael Holden

BAGHDAD, Feb 2 (Reuters) - Iraq's prime minister vowed on Saturday that attacks by two female bombers which killed 99 people in Baghdad would not derail improved security, but angry residents demanded the government do more to protect them.

Nuri al-Maliki said Friday's nearly simultaneous bombings at two crowded pet markets, the deadliest attacks in the city since April, would not herald a return to the savage violence that took Iraq to the brink of sectarian civil war.

The U.S. military said there were indications the women were mentally handicapped, and probably unaware they were being used as human bombs. It blamed Sunni Islamist al Qaeda for the attacks.

"I swear on the blood (of the victims), we will achieve all our goals in securing a stable Iraq. We will continue to ... crush the terrorists and target their strongholds," Maliki said in a statement.

Highlighting that Iraq faced serious security challenges across the nation, Maliki went to the northern city of Mosul and said a vital offensive was about to begin there after attacks that were also blamed on al Qaeda.

"The battle that our armed forces will engage in will tear out terrorism, criminal gangs and outlaws in this province," Maliki said of operations in Mosul, where a blast on Jan. 23 killed up to 50 people and wounded 220.

The attack at the Ghazil pet market in central Baghdad on Friday killed 62 people and wounded 129, just minutes after another blast killed 37 and wounded 67 at a bird market in southern Baghdad, police said.

Iraq's military said the bombs were detonated by remote control. Major-General Jeffery Hammond, the commander of U.S. troops in Baghdad, told reporters there were indications the two women were mentally impaired.

"It appears the suicide bombers were not willing martyrs, they were used by al Qaeda for these horrific attacks," he said.

Al Qaeda in Iraq, blamed by the U.S. military for most other large-scale bombings, has increasingly used women wearing suicide vests to carry out strikes after increased security and protective concrete walls made car bombings more difficult.

Hammond suggested using unwitting bombers could be a new tactic to circumvent tougher security measures.

"These two women were likely used because they didn't understand what was happening and they were less likely to be searched," he said.

GRIEF AND ANGER

As grieving relatives buried the dead, some Iraqis said the government was partly to blame.

"The two coordinated bombings proved the failure of government in maintaining peace in Baghdad," said teacher Basim Abdul-Ameer, 30, whose brother was wounded at the Ghazil market.

Friday's death toll was the worst in Baghdad since April 18, when multiple car bombings killed 191 people around the city.

The scale of the devastation seems to have shattered growing confidence among Iraqis that their streets were getting safer.

The Ghazil market, which opens only on Friday, sprawls into side streets and is difficult to secure. Local police said they would take steps to surround it with protective walls.

Ra'eed Hussain, 34, who normally takes his young son there, said he would not return until security improved.

"We need really thorough checks, especially of women wearing black abayas who could hide something underneath," said Hussain, referring to black robes that many older women in Iraq wear.

The attacks raise questions for the U.S. military, which has begun to reduce troop levels following a big drop in violence.

Attacks have fallen by 60 percent across Iraq since last June when 30,000 extra U.S. troops became fully deployed.

Troop levels will fall to around 135,000 by the middle of the year when more than 20,000 combat soldiers are withdrawn. (Additional reporting by Ahmed Rasheed and Aws Qusay; Editing by Dean Yates and Michael Winfrey)

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