The disease has affected about 100 US troops deployed in and around Iraq and left two of them dead, defence officials said in Washington on Friday.
"If the teams determine that the cases are unusual in any way, they will make preventive or corrective recommendations," the office of the Army surgeon-general said in a statement.
Cases have been noticed since 1 March when US troops entered the region in preparation for the invasion. Fifteen of the cases were so severe that they required ventilator support to remain alive.
Three US soldiers fell gravely ill with pneumonia in March, three more in April, two in May, three again in June and four in July, according to the Army.
So far, evidence does not support the theory of a local epidemic.
"Additionally, there is no evidence that any of the pneumonia cases being investigated have been caused by exposure to chemical or biological weapons, severe acute respiratory syndrome (SARS), or environmental toxins," his office added. A medical team is flying in to Iraq to determine the reasons.
|It has not been caused by exposure to chemical or biological weapons, SARS or environmental toxins|
The pneumonia scare is the latest setback for US forces in Iraq. Troop casualties on the ground are on the rise and resentment against occupation is increasing. In Najaf, Shia Iraqi cleric Moqtada Sadr launched his latest attack against the occupation troops during his Friday sermon.
Sadr took the US soldiers to task over heavy-handed tactics in their security sweeps.
Speaking to a gathering of around 100,000, Sadr asked, "Must we accept that Iraqis are humiliated and dragged helplessly along the ground by these soldiers? We demand that they be judged according to the Sharia (Islamic law)."
In continuing anti-occupation attacks, one US soldier died of gunshot wounds on Friday. He was with the First Armoured Division.
The soldier was standing outside when a bullet, fired from a celebrating Iraqi struck him, said a US Central Command statement. It happened late on Thursday.
He is the 52nd US soldier to have been killed in fighting in Iraq since May 1 when Washington declared “major combat operations” over.
At least another 59 have died in what the US calls non-combat deaths.