A soldier was knelt in morning prayer in a tiny sentry post when insurgents came up from behind and decapitated him. Another three were burnt to death in a vehicle, the sources said.
The killings came after the Iraqi army arrested around 60 people in sweeps in the town of Bohrouz, south of Baquba, where the insurgency has popular support, said local resident Akeel Mateb.
The al Qaeda linked group of Jordanian fugitive Abu Musab al-Zarqawi claimed responsibility for the attack in a statement posted on the Internet.
In another attack, seven policemen were killed and 15 wounded in a suicide car bomb outside a police station in Baiji, home to Iraq's largest oil refinery, a senior police officer said.
Baiji and Baquba fall respectively within Salahuddin and Diyala, two of four provinces where US and Iraqi officials fear insurgents will launch spectacular attacks aimed at deterring the local Sunni Muslim populace from voting.
The top US military official in Iraq, General George W. Casey, said on Monday that despite the coalition's best efforts, violence was unavoidable on election day and the level of bloodshed would likely continue unabated even after the polls.
Iraq's Deputy Prime Minister Barham Saleh delivered a similar prognosis, telling AFP the Iraqi government was "prepared for the worst" but argued an "imperfect" poll was better than none at all.
With much riding on their performance, Iraqi security forces demonstrated their prowess, killing 35 rebels and arresting 64 suspected militants in a span of 48 hours in an operation near the Sunni rebel city of Fallujah, the government said.
When US forces staged a massive onslaught against Fallujah in November, many wanted fighters were believed to have fled the insurgent hub, west of Baghdad, beforehand and regrouped elsewhere. West of Fallujah, a suicide car bomb exploded Monday afternoon and insurgents then fired guns and rocket-propelled grenades at marines, causing US casualties, a marine spokesman said.
There was no further word on the casualties. Two beheaded corpses of Iraqi soldiers were also found in Ramadi, where rebels regularly impose the rule of the gun on residents.
Insurgents also pressed their campaign of intimidation south of Baghad Sunday when 13 soldiers and two finance ministry employees were killed in a massive ambush by insurgents on a road linking the capital to the south-eastern province of Kut, medical and security sources said.
The US military also announced Monday that it had killed seven insurgents over a span of 24 hours in the main northern city of Mosul, where the January 30 polls are expected to be disrupted by relentless rebel attacks.
A member of the city's council who was also an important tribal leader was assassinated late Sunday by gunmen, an official from the governorate said.
Around the globe, members of the Iraqi diaspora were registering to take part in the milestone polls.
Up to 150,000 Iraqis living in the United Kingdom were expected to register in the scheme organised by the International Organisation for Migration in 14 countries.
Registration was also underway among Australia's 90,000-strong Iraqi community and in the Netherlands, where an estimated 45,000 Iraqis live.
On the diplomatic front, UN Secretary General Kofi Annan on Monday said the world body would be willing to expand its role in Iraq if conditions permit after this month's anticipated election.
And the Netherlands confirmed Monday it would pull its soldiers out of Iraq on March 15 from the southern province of Al-Muthanna.
The Iraqi Catholic archbishop of Mosul, Iraq, was kidnapped on Monday in what the Vatican called an "act of terrorism".
"We have received news of the kidnapping of the ... Archbishop of Mosul, Basile Georges Casmoussa," Chief Vatican spokesman Joaquin Navarro-Valls told Reuters. He gave no details of the abduction.
"The Holy See deplores this act of terrorism in the firmest manner and demands that the worthy pastor is swiftly freed unharmed to continue to carry out his ministry," the spokesman said.
Casmoussa, 66, was believed to be the highest ranking Catholic to be abducted in Iraq, where the local church has been the target of a bombing campaign aimed at intimidate the tiny Christian minority.