U.S. Marines (search) broke down doors and raided houses Monday on the second day of an offensive aimed at cracking down on insurgent activity in several troubled cities west of Baghdad.

Militants announced they were releasing a pair of kidnapped Indonesian journalists missing since last week in a new video delivered anonymously to Associated Press Television News. It was not possible to verify the video's authenticity or determine when it was made. The hostages whereabouts were not known.

Meanwhile, Shiites and their clergy-backed United Iraqi Alliance (search) met Monday in Baghdad to renew discussions over who their prime ministerial candidate would be. But instead of narrowing the choices down, the field of potential candidates has grown to four, maybe even five, insiders said.

The two most prominent candidates have been former Pentagon favorite Ahmad Chalabi (search), a secular Shiite, and Ibrahim al-Jaafari, interim vice president. The race may get more complicated following reports the Shiite's initial pick for prime minister, Finance Minister Adel Abdul-Mahdi, who has close ties to Iran, could become a compromise candidate.

Six explosions boomed through the capital before midday. The cause of the blasts was not immediately known. Footage from Associated Press Television News showed U.S. troops treating an American soldier apparently injured in one of the blasts, which overturned a Humvee in the southern Doura neighborhood.

In Ramadi, U.S. Marines fanned out across the city, setting up checkpoints, searching cars and sealing off sections of the city to prevent people from entering or leaving as they carried out raids. Soldiers began the operation on Sunday, slapping a nighttime curfew on the city.

Iraqi Maj. Abdul Karim al-Faraji said troops detained a prominent Sunni Muslim sheik, Mohammed Nasir Ali al-Ijbie, who heads the al-Bufaraj tribe, along with 12 of his relatives.

As the Shiite majority prepared to take control of the country's first freely elected government, tribal chiefs representing Sunni Arabs in six provinces issued a list of demands — including participation in the government and the drafting of a new constitution — after previously refusing to acknowledge the vote's legitimacy.

"We made a big mistake when we didn't vote," said Sheik Hathal Younis Yahiya, 49, a representative from northern Nineveh. "Our votes were very important."

He said threats from insurgents — not sectarian differences — kept most Sunnis from voting.

Sunnis make up 20 percent of Iraq's population of 26 million; Shiites make up 60 percent.

Gathering in a central Baghdad hotel on Sunday, about 70 tribal leaders from the provinces of Baghdad, Kirkuk, Salaheddin, Diyala, Anbar and Nineveh, tried to devise a strategy for participation in a future government. There was an air of desperation in some quarters of the smoke-filled conference room.

"When we said that we are not going to take part, that didn't mean that we are not going to take part in the political process. We have to take part in the political process and draft the new constitution," said Adnan al-Duleimi, the head of Sunni Endowments in Baghdad.

Meanwhile, a powerful Sunni organization believed to have ties with the insurgents sought to condemn the weekend attacks largely aimed at Shiites that left nearly 100 Iraqis dead.

"We won't remain silent over those crimes which target the Iraqi people — Sunnis or Shiites, Islamic or non-Islamic," Sheik Harith al-Dhari, of the Association of Muslim Scholars, told a news conference.

Iraqis, he said, should unite "against those who are trying to incite hatred between us."

They include Iraq's leading terror mastermind, the Jordanian-born Abu Musab al-Zarqawi. In a letter to Osama bin Laden found on a captured al-Qaida courier last year, al-Zarqawi proposed starting a civil war between Iraq's Sunni and Shiite Muslims.

Shiite politicians have promised not to allow Friday and Saturday's bloodshed to escalate into a civil war. A series of attacks, including eight suicide bombings, killed 91 people and injured dozens as Iraqi Shiites commemorated the seventh century death of a leader of their Muslim sect.