Shiite religious parties captured the biggest number of parliament seats in last month's election but not enough to govern without partners, according to results released Friday.

Sunni Arabs scored major gains, opening the door to a greater role in government for the community at the heart of the insurgency.

The announcement by the election commission launched a period of tough bargaining among Shiite, Sunni and Kurdish factions to form a government, which U.S. officials hope can win the trust of the disaffected Sunni Arab minority so U.S. and other foreign troops can go home.

Those hopes were buoyed by a threefold increase in the number of Sunni Arabs in parliament, a move that increases their chances for important posts in the new government. Many Sunni Arabs boycotted the January 2005 election, enabling Shiites and Kurds to dominate the government -- sharpening sectarian tensions and fueling the insurgency.

"Iraq is in a very delicate situation and has problems with foreign forces, so we need a government of national unity that should include the Shiite list, Kurdish coalition and the Sunnis," Kurdish politician Mahmoud Othman said. "When everyone is together, the problems will be less."

However, the results of the Dec. 15 balloting also affirmed the power of religiously based politics in a country wracked by sectarian violence.

An avowedly sectarian ticket headed by Ayad Allawi lost seats, although the former interim prime minister himself won election. Deputy Prime Minister Ahmad Chalabi, a sectarian Shiite once seen as America's choice to rule after Saddam Hussein, failed to win a seat.

According to the results, which must be certified within two weeks, the Shiite United Iraqi Alliance won 128 of the 275 seats, down from the 146 it held in the outgoing parliament. Two major Sunni Arab groups took a total of 55 seats. Only 17 Sunni Arabs were in the old legislature.

Some Sunni gains were at the expense of the Kurds. An alliance of two Kurdish parties, allied with the Shiites in the outgoing government, won 53 seats, down from the 75 they held in the old parliament. Minor parties accounted for the rest of the seats.

Under the new constitution ratified last October, parliament can approve a new prime minister and Cabinet by only an absolute majority of its members. However, the president and two vice presidents must win the votes of two-thirds of parliament members.

Results were delayed for a month because of charges by Sunni politicians and others of widespread fraud and other irregularities. The government agreed to a review by a commission of foreign experts, who concluded in a report Thursday that the election was flawed but essentially fair.

Despite their gains, some Sunni politicians expressed disappointment with the results, insisting they were victims of fraud. Nevertheless, they agreed to participate in parliament, and it appeared their objections were largely posturing.

"The dishonesty of the electoral commission is the main reason behind this disappointment because it is under the thumb of one list," Sunni politician Salman al-Jumaili said. "In all cases, we will deal positively with these results, take part in the coming parliament and government and present our challenges to the Iraqi judicial system."

An ally of Allawi, Saad Asem al-Janabi, blamed his ticket's losses on alleged favoritism in the election commission.

"This is a real disappointment for the democracy in Iraq because the electoral commission is biased, not independent and honest, and all its procedures served one list and party," he said, referring to the Shiites.

Sunni candidates profited in part by changes in the election law, which enabled voters to choose their representatives by district. In January 2005, seats were allocated based on the percent of the vote won by various tickets across the country.

That gave a huge advantage to parties representing the Shiite community, which forms about 60 percent of Iraq's 27 million people. The switch to a district system guaranteed Sunnis would be elected in overwhelmingly Sunni areas regardless of voter turnout.

Many influential Sunnis also concluded that the January boycott proved a disaster to their community and encouraged their followers to participate in the December vote.

Bringing moderates from the Sunni Arab community into the political process had been a U.S. goal in the recent months, and U.S. officials hailed the election as an important step toward stability and progress in this country ravaged by years of war and dictatorship.

Attention now focuses on negotiations for the new government. U.S. Embassy spokesman Tom Casey said Washington would "continue to support efforts of Iraq's political leaders to form a unity government."

U.S. military officials fear a surge in violence if the talks do not go smoothly.

"We anticipate that attacks will be considerably elevated," said Lt. Col. Brian Winski, commander of the 1st Squadron 61st Cavalry. He said suicide car bombs and more roadside explosives were a major concern.

Politicians have four days to contest the results, which were largely in line with preliminary returns. Officials then will have 10 days to study any complaints before they certify the results and parliament convenes to appoint a new government.

Heightened security measures accompanied the announcement of the election results, with thousands of police manning checkpoints and patrolling roads leading in and out of Baghdad and Anbar, Diyala, Najaf and Mosul provinces.

The closures were expected to continue Saturday. Only Iraqis returning from a Muslim pilgrimage in Saudi Arabia will be able to enter the provinces, said army Capt. Jassim al-Wahish.

U.S. and Iraqi troops also launched a major military operation in southern Baghdad's Dora neighborhood at dawn Friday in the hunt for two local insurgent leaders believed to control several hundred militants, including non-Iraqi Arabs, said Iraqi Army Gen. Mehdi al-Gharawi.

A series of loud explosions and bursts of machine gun fire were heard as the troops hunted suspected militant chiefs Sheik Fathi al-Jibouri and relative Abu Aisha al-Jibouri, al-Gharawi told The Associated Press.