US military spokesman Corporal Todd Pruden said three US soldiers were killed in northern Iraq when troops from the 101st Airborne Division came under small arms and rocket-propelled grenade fire.
Pruden did not specify where or at what time.
But Aljazeera's correspondent in Mosulk Yasir Abu Hilala said: “The new resistance attack was carried out in Al-Wihda area.”
“Eyewitnesses said four US forces were killed,” he reported.
This can not be confirmed, but some eyewitnesses believe that all transferred bodies, including those with injuries, are dead,” he added.
The US 101st Airborne Division specialises in tracking members of the former Iraq government.
The deaths take the toll of US soldiers killed by Iraqi resistance fighters to 45 since United States President George Bush declared “major combat operations” in Iraq over on 1 May.
US AND BRITISH TROOPS KILLED:
US 158 (44)
US 79 (57)
British 29 (4)
Civilians Between 6,073 and 7,782*
Figures in brackets refer to casualties after 1 May
# = U.S. military estimates relating only to fighting in or
near Baghdad. No other figures available.
* = Figure compiled on Web site www.iraqbodycount.net, run by academics and peace activists, based on incidents reported by at least two media sources.
Nearly 80 more soldiers have died in what the US calls "non-hostile" incidents.
In the other encounter in Baghdad, US troops opened fire on an Iraqi civilian vehicle after it refused to stop at a military checkpoint in Baghdad.
The car burst into flames, leaving the two occupants dead, witnesses said.
"The car was speeding in the direction of the Americans. I tried to warn them, I waved my hands. They hit the brake as they bumped into the barbed wire fence," said Khodayir Haidar Ali, 42, who was wounded in the incident.
"Within one minute of the shots the vehicle burst into flames," he added.
It was not clear if the blast was the result of the US gunshots igniting gasoline or of explosives carried inside the car, believed to be a taxi.
Three separate witnesses at the site of the blast in central Baghdad said they believed the men were resistance fighters.
An AFP correspondent at the scene said the charred torsos of two people were still slumped inside the burned debris of vehicle.
Another witness said the incident followed a search by US troops of the Abd al-Qadir al-Gilani mosque earlier in the day.
"The Americans arrived at around 6:00 am (0200 GMT) and searched the mosque, they were looking for someone," said one witness, who asked to be identified only as Afeli.
"Many people tried to stop the car but it did not take notice," he said, adding: "We're not sure if the blast was from inside the car or from the gunfire."
Witnesses had earlier said the incident was an attack on a US military vehicle, describing what they believed to be a rocket-propelled grenade attack preceded by small arms fire. They said it occurred at around 7:30 am (0330 GMT).
A US military spokesman said he was unable to immediately confirm the incident.