(CBS/AP) The U.S. military tells CBSNews.com that 26 American service members have been killed in action in Iraq in the past week alone, including three soldiers who were killed by a single roadside bomb attack reported Tuesday.

Most recently reported were the three Task Force Marne soldiers killed Saturday when a roadside bomb struck their convoy south of Baghdad, according to a brief statement that provided no more details.

One Multi-National Division - Baghdad soldier was killed and another wounded Monday when their vehicle was targeted by an armor-piercing explosively formed penetrator, or EFP, in a western section of the capital, the military said separately.

Lt. Col. Rudy Burwell, a military spokesman based at Camp Victory in Baghdad, told CBSNews.com that a total of six troops were killed in action Monday.

The U.S. military has accused Iran of supplying Shiite extremists with EFPs to step up attacks against American forces. Tehran denies the allegations.

The deaths raised to at least 3,678 members of the U.S. military have died since the Iraq war started in March 2003, according to an Associated Press count.

August has begun with a wave of U.S. troop deaths in Iraq, on the heels of a relatively low death toll in July, which was cited by commanders as an indication that that the build-up of American troops in and around Baghdad was reducing violence.

The military reported Monday that four U.S. soldiers had died from wounds suffered in a combat explosion in Diyala province north of Baghdad earlier that day. Twelve others had minor injuries and returned to duty.

The military statement announcing the deaths gave no other details and said identities of the victims were being withheld until family could be notified.

Earlier Monday the military said one soldier was killed during fighting in eastern Baghdad a day earlier. Two soldiers were wounded in the fighting.

Meanwhile Iraq's political crisis worsened Monday as five more ministers announced a boycott of Cabinet meetings, leaving the embattled prime minister's unity government with no members affiliated with Sunni political factions.

Also Monday, a suicide bomber killed at least 28 people in a northern city, including 19 children, some playing hopscotch and marbles in front of their homes.

The new cracks in Prime Minister Nouri al-Maliki's government appeared even as U.S. military officials sounded cautious notes of progress on security, citing strides against insurgents linked to al Qaeda in Iraq but also new threats from Iranian-backed Shiite militias.

Despite the new U.S. accusations of Iranian meddling, the U.S. and Iranian ambassadors met Monday for their third round of talks in just over two months. A U.S. embassy spokesman called the talks between U.S. Ambassador Ryan Crocker and his counterpart, Hassan Kazemi Qomi, "frank and serious."

But it was al-Maliki's troubles that seized the most attention.

In other developments:

The Cabinet boycott of five ministers loyal to former Iraqi leader Ayad Allawi left the government, at least temporarily, without participants who were members of the Sunni political apparatus, a deep blow to the prime minister's attempt to craft reconciliation among the country's majority Shiites and minority Sunnis and Kurds.

The defense minister is from a Sunni background but has no political ties and was chosen by al-Maliki.

The Allawi bloc, a mixture of Sunnis and Shiites, cited al-Maliki's failure to respond to its demands for political reform. The top Sunni political bloc already had pulled its six ministers from the 40-member Cabinet of al-Maliki, a Shiite, last week.

Iraqi President Jalal Talabani, who has been trying to broker the Sunni bloc's return in a bid to hold the government together, met Monday with Crocker and a White House envoy.

In Washington, State Department spokesman Sean McCormack said the United States was working well with the al-Maliki government, but he did not give the kind of enthusiastic endorsement that President George W. Bush and his aides once did.

"There's a very healthy political debate that is going on in Iraq, and that is good," McCormack said. "It's going to be for them (the Iraqi people) to make the judgments about whether or not that government is performing."

Lawmaker Hussam al-Azawi, of the bloc loyal to Allawi, said the boycott began with Monday's Cabinet meeting. The ministers intend to continue overseeing their ministries.

"We demanded broader political participation by all Iraqis to achieve real national reconciliation ... and an end to sectarian favoritism," al-Azawi said.

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