Militants struck back Sunday in their first major blow against a U.S.-led security clampdown in Baghdad with car bombs that killed at least 63 people, left scores injured and sent a bloody calling card to officials boasting that extremist factions were on the run.

The attacks in mostly Shiite areas — twin explosions in an open-air market that claimed 62 lives and a third blast that killed one — were a sobering reminder of the huge challenges confronting any effort to rattle the well-armed and well-hidden insurgents.

Instead, it was the Iraqi commanders of the security sweep feeling the sting.

Just a few hours before the blasts, Lt. Gen. Abboud Qanbar led reporters on a tour of the neighborhood near the marketplace and promised to "chase the terrorists out of Baghdad." On Saturday, the Iraqi spokesman for the plan, Brig. Gen. Qassim Moussawi, said violence had plummeted 80 percent in the capital.

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Prime Minister Nouri al-Maliki condemned the bombing as a desperate act by "terrorists" and "criminals" who sense they are being squeezed.

"These crimes confirm the defeat of these perpetrators and their failure in confronting our armed forces, which are determined to cleanse the dens of terrorism," al-Maliki said in a statement.

U.S. military chiefs have been much more cautious. They have insisted the security drive, begun last week, may take months to make clear gains and that counter-punches from militants were likely every step of the way.

The ones dealt Sunday came from the militants' favored weapon of the moment: parked cars rigged with explosives.

The first blast tore through a produce market in the mostly Shiite area of New Baghdad, toppling the wooden stalls and leaving pools of blood and vegetables trampled in the chaos. Minutes later, another car bomb exploded near a row of stores.

More than 129 people were injured, including many women who were shopping, said police and rescue officials who spoke on condition of anonymity because they are not authorized to speak to the media.

Victims were carried to hospitals on makeshift stretchers or in the arms of rescuers.

Another car bomb in the Shiite enclave of Sadr City left at least one dead and 10 wounded, police said.

It was by far the deadliest day since the security sweeps began last week. On Thursday, a string of car bombs killed seven civilians on the first full day of the house-to-house searches for weapons and suspected militants.

The U.S.-led teams have faced limited direct defiance as they set up checkpoints and comb neighborhoods. But that could change as they move into more volatile sections. The next could be Sadr City, a stronghold of the Mahdi Army militia loyal to radical Shiite cleric Muqtada al-Sadr.

U.S. soldiers pressed closer to Sadr City and the reception changed noticeably. In previous days, Shiite families opened their doors to welcome the troops — feeling that the American presence would be a buffer against feared attacks from Sunni militia.

On Sunday, in areas closer to Sadr City, parents slapped away the candy and lollipops given by GIs.

"The Baghdad security plan is very important to push Iraq ahead," said Haider al-Obeidi, a parliament member from the Dawa party of the prime minister al-Maliki.

An officer at the Defense Ministry vowed: "We have plans to drain the breeding grounds of terrorists ... We need only some time and patience for this goal." He spoke on condition of anonymity because he is not authorized to make public comments.

The Baghdad crackdown has sent ripples to all corners of the country. The borders with Iran and Syria — shut for three days as the plan got under way — reopened Sunday. But new and strict rules will apply.

Moussawi was quoted in the Azzaman newspaper as saying the crossing points to the two nations would be open for only several hours a day and under "intense observation."

The United States and allies claim Iraqi militants receive aid and supplies from Iran, including parts for lethal roadside bombs targeting U.S. forces. Iran denies any role in trafficking weapons.

In Tehran, Syrian President Bashar Assad held talks with Iranian leaders, including President Mahmoud Ahamedinejad. They are generally on opposing sides of Iraq's sectarian divide: Iran backing the majority Shiites and Syria seen as a key supporter of Sunnis.

But Iran denied U.S. and Iraqi government reports that the cleric al-Sadr has crossed over from Iraq. Conflicting reports about his whereabouts have surfaced for nearly a week.

"No, he is not in Iran," Mohammad Ali Hoseini, spokesman for the ministry, told journalists during a regular press briefing in Iran's first comment on the issue. "The report is baseless and a kind of psychological warfare against Iran by the U.S. to put more pressure on Iran."

Al-Sadr's Mahdi Army is widely believed to receive Iranian money and weapons — as do other Shiite groups here — but his political wing is part of Iraq's U.S.-backed government.

On Saturday, U.S. Secretary of State Condoleezza Rice told al-Maliki that the security push needs to "rise above sectarianism" and tackle both Sunni and Shiite districts in order to be credible, said an Iraqi official who spoke on condition of anonymity because he was not authorized to speak to media.

"None of us knows the full story of precisely what the militias are doing," Rice said. "But if there is a diminution in the violence as a result, if they have decided that they are not going to challenge the Baghdad security plan, then the use of that time for good purposes could make the situation much more stable."

Two more U.S. soldiers have been killed in action, the U.S. military said. Both were killed Saturday: one by a grenade in a northern neighborhood of Baghdad; the other from gunfire north of the city, the statement said.

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