BAGHDAD - A suicide bomber slipped past security barriers to kill 12 people Thursday, the latest in a wave of attacks that have shown the resilience of insurgents in the face of a US-led crackdown on major violence in Iraq's capital.

An injured Iraqi man lies at a hospital after a suicide car bomber crashed into an Iraqi police checkpoint, killing 33 and injuring 75 at an entrance to Sadr City, the capital's biggest Shiite Muslim neighborhood, in Baghdad, Iraq, Wednesday, April 18, 2007. [AP]

The attack in a mostly Shiite district showed yet again the ability of insurgents to penetrate Baghdad's heavy security presence, a day after more than 230 people died in the worst spasm of mass killings since President Bush announced his plan in January to increase American troop levels in Iraq by 30,000.

Iraq's prime minister, Nouri al-Maliki, said an "open battle" was being waged for control of his nation.

Thursday's attacker blew himself up next to a fuel tanker within 500 yards of Iraqi President Jalal Talabani's home in the Karradah district. Talabani, a Kurd, was not believed to have been the target. Two Iraqi soldiers were among the dead, and 34 people were wounded, police said.

US commanders urged patience, saying the nine-week operation was still just beginning. Three of the five brigades Bush ordered into Iraq to stem Baghdad violence have arrived, bringing the US forces in the country to 146,000. Officials want the rest in place by June for a total of 160,000.

But already insurgents have exploited the operation's vulnerabilities. One week ago, a suicide bomber penetrated several layers of security to hit inside parliament, in the heart of US-guarded Green Zone, killing an Iraqi lawmaker. The same day, a truck bomber collapsed a more than 50-year-old bridge, killing 11 people and sending cars careening into the Tigris River.

At the Pentagon, a top general predicted the pattern was likely to continue.

"We saw an initial drop in their (militants') activity" after the start of the Baghdad security operation, said Maj. Gen. Michael Barbero, an operations official for the Joint Chiefs of Staff. "And now lately, we've seen an increase ¡ª the bridge, this.... It's action on our part and now we're seeing the reaction on their part. And it will be like that until we can defeat these forces."

Anthony Cordesman, an Iraq expert at the Washington-based Center for Strategic and International Studies, called the dramatic breaches of security "exercises in symbolism more than terror."

"What they're really designed to do is to show Iraq, the region, the US Congress that (the Baghdad security clampdown) is not working," he said.

Cordesman said such bombings can never be fully prevented in urban areas.

"People have gotten the impression that we can make (a city) leakproof," Cordesman said. "We can't do it there; we can't do it here."

Despite new barricades and checkpoints erected as part of the security crackdown, a fraction of the cars in Baghdad ¡ª a city of 6 million residents ¡ª are searched at all. Many of the suicide car bombs explode at the checkpoints, either targeting Iraqi troops or detonating a moment before they are discovered.

Some local media have suggested that Sunni insurgents have secretly stockpiled explosives in Shiite areas, and are now rigging their cars with bombs very close to their targets to avoid driving long distances and risking security checks.

Lt. Col. Christopher Garver, a US military spokesman, told The Associated Press the American military strategy was two-pronged: raiding car-bomb factories on the outskirts of Baghdad, and clearing weapons stashes hidden in dense urban areas inside the capital.

"We want to close down access to the city, but we also want to be inside these neighborhoods to find these caches of explosives. If the final assembly exists inside the city, that's what our clearing operations will be targeting," he said.

But he said the strategy would not be fully implemented until June 1.

"We don't have all the troops for the surge ¡ª we're only at three of five brigades so far. It's not fully in place," Garver said. "Still, I can't say if we had those two brigades, yesterday wouldn't have happened. This enemy is adaptive."

Thursday's bombing hit hours before US Defense Secretary Robert Gates arrived on an unannounced visit to warn Iraqi leaders that the US commitment to a military buildup there is not open-ended.

Gates said the political tumult in Washington over financing the war shows that both the American public and the Bush administration are running out of patience.

"I'm sympathetic with some of the challenges that they face," Gates said of the Iraqis. But, he said, "the clock is ticking."

Al-Maliki, the prime minister, said militants had "proven their spite by targeting humanity."

"It is an open battle and it will not be the last in the war we are fighting for the sake of the nation, dignity, honor and the people," he said at a ceremony marking the 50th anniversary of the founding his Islamic Dawa Party.

"This is Iraq. They sabotage and we build and continue the reconstruction," al-Maliki said defiantly.

Insurgents were equally defiant.

A Sunni insurgent coalition posted Web videos on Thursday naming the head of al-Qaida in Iraq as "minister of war" and showing the executions of 20 men it said were members of the Iraqi military and security forces.

The announcement unveiling an "Islamic Cabinet" for Iraq appeared to have multiple aims. One was to present the Islamic State of Iraq coalition as a "legitimate" alternative to the US-backed, Shiite-led government ¡ª and to demonstrate that it was growing in power despite the US military push against insurgents. It also likely sought to establish the coalition's dominance among insurgents after an embarrassing public dispute with other Iraqi Sunni militants.

At least 46 Iraqis were killed or found dead nationwide Thursday.

The US announced three more troop deaths ¡ª two soldiers killed Wednesday by a roadside bomb north of the capital, and another soldier killed the same day in a small arms fire attack in southwest Baghdad.

Two British soldiers were killed and three others wounded Thursday by an explosion in southeastern Iraq. The attack occurred in Maysan province, a day after British troops transferred control of the area to Iraqi forces.

Many of the more than 230 Iraqis killed or found dead a day earlier were buried in quiet ceremonies before Thursday's noon prayer, according to Muslim tradition. Other bodies were in refrigeration containers, still unidentified, at morgues across Baghdad.

In Baghdad's Sadr City district, relatives flocked to Imam Ali Hospital to claim the bodies of loved ones. A man held his shirt over his mouth and nose as he moved past decaying bodies. Nearby, four men loaded a casket onto a minibus.

Collective wakes were held for multiple victims in huge tents erected in narrow alleys and at mosques close to the blast sites. Onlookers gathered around a crater about three yards wide, left by the force of one explosion.

One of them, 38-year-old Akram Abdullah, who owns a clothing shop about 200 yards away, fell to his knees in tears.

"It's a tragedy ¡ª devastation covers the whole area. It's as if a volcano erupted here," said Abdullah, the father of three boys.

"Charred dead bodies are still inside the twisted cars, some cars are still covered with ashes," he said, describing the scene before him in a phone interview.

Abdullah, whose shop was damaged by flying shrapnel, said he took part in 18 funerals Thursday morning. "I cried a lot," he said.