Insurgents killed three more American soldiers in the Baghdad area Sunday and fired mortars near the Defense Ministry in a spree of violence that killed at least 27 Iraqis as politicians began work on forming a new government.

The largest Sunni Arab party raised new allegations of sectarian killings — one of the most urgent issues facing the new Iraqi leadership.

U.S. Ambassador Zalmay Khalilzad said the next government must decommission sectarian militias and integrate them into the national armed forces, warning that the armed groups represent the "infrastructure for civil war."

Sunday's deaths raised to eight the number of U.S. troops killed the past two days.

At least 61 American service members have died in April, putting it on track to pass January — with 62 — as the deadliest month this year. It represents a jump over March, which with 31 deaths was the lowest monthly toll for the Americans since February 2004.

The three solders were killed Sunday when their vehicle hit a roadside bomb northwest of capital, the U.S. command said.

Twenty-seven Iraqis also died in other violence Sunday, including seven who were killed when three mortars hit just outside the heavily guarded Green Zone in the capital, not far from Iraq's Defense Ministry. Police Lt. Maitham Abdul-Razzaq said it was hard to identify the seven dead because the powerful blasts and shrapnel severed their limbs and destroyed their identification cards.

At least eight other mortars or rockets exploded at about the same time on the other side of the Tigris River in central Baghdad, without causing injuries, police said.

In the evening, another mortar hit a home in southern Baghdad, killing a man and wounding two of his family members. Driveby shootings in a nearby district gunned down a woman schoolteachers outside her home and a car mechanic in his shop.

The violence underlined the challenge as prime minister-designate Jawad al-Maliki began on Sunday the tough task of pulling together a Cabinet out of Iraq's Shiite, Sunni and Kurdish parties.

Al-Maliki, a Shiite, has 30 days to do it, but the parties are under enormous pressure — from Americans and even from Shiite religious leaders — to move quickly and put aside the often intractable haggling over ministries.

The United States is hoping the new government will unify Iraq's bitterly divided factions behind a program aimed at reining in both the Sunni-led insurgency and the Shiite-Sunni killings that flared out of control during months without a stable government.

Khalilzad, a key player in tortuous political negotiations since Iraq's Dec. 15 elections, repeated his call for the quick creation of a Cabinet made up of "competent" ministers — implying those chosen for their skills and not sectarian or political ties.

He also issued a strong warning Sunday against militias, calling them "a serious challenge to stability in Iraq to building a successful country based on rule of law."

"There is a need for a decommissioning, demobilization and reintegration plan for these unauthorized military formations," he told a press conference with Iraqi President Jalal Talabani in the northern city of Irbil.

Sunni Arabs say Shiite militias have infiltrated the Interior Ministry — controlled by the biggest Shiite party — and have used death squads to kill Sunnis. Sectarian violence has flared since the Feb. 22 bombing of a Shiite shrine in Samarra, north of Baghdad.

But the killings have gone both ways. Police said the bodies of six Shiites were found Sunday in the mainly Sunni district of Azamiyah in Baghdad, their hands and legs bound and their bodies showing signs of torture. Two more — their identities unknown — were found in a mixed district south of Baghdad.

The head of the Azamiyah district council, Sheik Hassan Sabri Salman, said relatives on Sunday identified the bodies of 14 Sunnis kidnapped last week. The bodies, he said, were handcuffed with signs of torture. Police did not confirm the deaths.

The Iraqi Islamic Party, the main Sunni faction in parliament and a likely participant in the next Cabinet, warned of "the repercussions of sectarian cleansing." It urged the new government to stop "the criminal gangs" involved in the killings.

Control of the Interior Ministry will be a key question in forming the government. The Shiite Supreme Council for the Islamic Revolution in Iraq — which currently holds it — appeared to be under pressure to give it up. SCIRI ran the feared Badr Brigade militia during Saddam Hussein's rule but insists the group has given up their arms, a claim many Sunnis reject.

One name being touted to take the post was Qassim Dawoud, an independent Shiite legislator who held a security positions in the administration of former Prime Minister Ayad Allawi and is not connected to militias.

But uprooting militia power will be difficult for any government. Al-Maliki has vowed to implement a law that would integrate them into the security forces, but there is little guarantee that the forces — once in the army or police — would then drop their loyalty to their former sectarian commanders.

The Sunni parties have said they can work with al-Maliki, he must overcome a reputation as a hard-line Shiite partisan.

The new prime minister was known for his sharp anti-Sunni comments during bitter negotiations over the constitution least year and during his work in the commission purging members of the ousted Baath Party from the government and military.

Khalilzad called al-Maliki "is a tough guy, tough-minded as well. He has been very tough on the issue of terrorism."

"However, his statements before he became the prime minister, or became the nominee — it will be different once he's in office," the ambassador told CNN's "Late Edition."

Despite the violence, some Iraqis in Baghdad said they were encouraged by the legislators' success in finally beginning to form a new government.

"It took too long, but it is a good step on the right direction. It could be a springboard for the stability of this country," said Hussein Farij in Tahrir Square in central Baghdad.

"We pin a great hope on the formation of a new government. It must heal our country's many wounds," said Majeed Hameed.