Iraq's prime minister says an "open battle" is being waged for control of his nation, as a homicide bomber slipped past security barriers to kill 12 people in a surge of violence that has dealt a heavy blow to hopes for a U.S.-led military operation to pacify the capital.

The U.S. announced three more troop deaths -- two soldiers killed Wednesday by a roadside bomb north of Baghdad, and another soldier killed the same day in a small arms fire attack in a southwestern area of the capital.

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

Prime Minister Nouri al-Maliki's assessment Thursday came a day after more than 230 people died in the worst spasm of mass killings since U.S. President George W. Bush announced his plan in January to increase American troop levels in Iraq by 30,000.

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

One week ago, a suicide bomber penetrated several layers of security to hit inside parliament, in the heart of U.S.-guarded Green Zone. An Iraqi lawmaker was killed and the country shaken.

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 below.

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 -- 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."

Al-Maliki said Thursday that 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.

Thursday's bombing hit when security would have been tightest, hours before U.S. Defense Secretary Robert Gates arrived on an unannounced visit. Gates said he was "sympathetic" to challenges the Iraqis face, but that "the clock is ticking."

The homicide bomber blew himself up next to a fuel tanker within half a kilometer (500 yards) of Iraqi President Jalal Talabani's home, in the mostly Shiite Karradah district where one of Wednesday's bombs exploded as well. Talabani, a Kurd, was not believed to have been the target.

The attack killed at least 12 people, including two Iraqi soldiers, wounded 34 people and set fire to the tanker, police said.

Britain's defense ministry also said two British soldiers died and three others were 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.

At least 3,314 members of the U.S. military have died since the Iraq war started in March 2003, according to an Associated Press count. The figure includes seven military civilians. The British military has reported 144 deaths.

Despite new barricades and checkpoints erected as part of the security crackdown, a fraction of the cars in Baghdad -- a city of 6 million residents -- 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 U.S. 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 -- 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."

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 laid in refrigeration containers, still unidentified, at morgues across Baghdad.

In Sadr City, 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 3 meters wide, left by the force of one explosion.

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

"It's a tragedy -- 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.

Meanwhile, the Islamic State of Iraq, a coalition of Sunni insurgents including Al Qaeda in Iraq, issued a video Thursday purporting to show the killing of 20 kidnapped Iraqi police and soldiers, shot in the head execution-style as they knelt in a row.

The group claimed to have abducted the 20 and threatened to kill them after 48 hours unless the government freed female prisoners and handed over police accused of rapes in the northern town of Tal Afar.

The authenticity of the six-minute video could not be independently confirmed, and the Iraqi government has denied that 20 police and soldiers were kidnapped.

Interior Ministry spokesman Brig. Gen. Abdul-Karim Khalaf said Thursday that the men in the video could not be identified and said the insurgents may have dressed up civilians to kill them.

In another video purportedly issued by the group, a man identified as a spokesman announced a "Cabinet" for the Islamic state in Iraq, naming the head of Al Qaeda in Iraq Abu Hamza al-Muhajer, also known as Abu Ayyub al-Masri, as "minister of war."