A roadside bomb on Tuesday also killed a local police chief in Baghdad, just hours before interim Prime Minister Iyad Allawi was due to return home after trying to win support from neighbouring countries to stabilise Iraq.
The suicide car bomb blast at a checkpoint outside the town of Baquba wounded six other Iraqi guardsmen, National Guard Lieutenant Mohamed al-Dulaimi said.
Major Deborah Stewart of the US 1st Infantry Division said the wounded had been evacuated to a nearby hospital.
The US military said two American soldiers were killed in a roadside bomb blast overnight on Baghdad's western outskirts.
And two US marines were killed in action in the violent Anbar province in the country's west. One died on Monday from wounds and the other on Tuesday, the US military said.
Early on Tuesday, a roadside bomb in Baghdad's upscale Mansour district killed the head of a local police station and wounded two of his bodyguards, police said.
The bomb exploded as Colonel Moayad Mahmoud Bashar, chief of the Mamoun police station, was driving past.
Allawi has spent the past 10 days visiting Iraq's Arab partners to shore up support for his government and seek help in stemming an insurgency that has disrupted reconstruction.
He especially sought to get Iraq's neighbours, including Syria, to tighten border controls to stop foreign fighters from crossing over to join the insurgency.
A government official said Allawi would arrive back on Tuesday afternoon.
One of his most pressing security challenges is a spiralling hostage crisis, which has forced the Philippines to withdraw troops and at least two foreign firms to pull out of Iraq.
TRUCKERS STILL HELD: Talks to free seven foreign truck drivers threatened with execution have stalled since Monday, mediators said. The three Indians, three Kenyans and an Egyptian were seized last month.
A tribal Sheikh trying to win their release said on Tuesday he was waiting to hear from the kidnappers, who have demanded their Kuwaiti employer leave Iraq and compensate families who suffered in US air strikes on Falluja.
"Negotiations are still stopped at the moment. I have no idea on the fate of the hostages," Hisham al-Dulaymi said.
There has been a surge in kidnappings since Manila pulled its troops out last month to save the life of a Filipino driver.
Some of the kidnappings have been carried out by groups linked to al Qaeda ally Abu Musab al-Zarqawi, who has claimed responsibility for some of the biggest suicide attacks in Iraq and the killing of several foreign hostages.