The headquarters compound returned fire, officers said, but casualty reports from the other side were not immediately available. A US Central Command spokesman in Qatar said the incident was under investigation. The clash near Nassiriyah, which destroyed at least six military vehicles, came amid renewed airstrikes overnight and early yesterday on Baghdad, mainly on the southern edge of the capital, home to the huge Al-Rasheed military camp.

One captain told the AFP reporter that US troops fired at one another in the confusion of the nighttime engagement. “It was friendly fire,” said the officer, who did not want to be named.

But an Iraqi military spokesman said the claims of friendly fire were “lies” and that Iraqi forces had inflicted the casualties. “Our forces carried out consecutive raids on enemy columns near Nassiriyah, causing deaths and injuries,” Hazim Al-Rawi told reporters in Baghdad.

Gen. Vincent Brooks, with the US Central Command in Qatar, confirmed that reports of a “blue-on-blue” firefight had been received and were being examined. US and British forces have suffered a string of deadly fratricide incidents since they launched their invasion of Iraq more than a week ago to disarm and topple the regime of Saddam Hussein. Despite technological advances which provide US soldiers with unprecedented vision of the battlefield, so-called “friendly fire” incidents persist in demoralizing troops and embarrassing commanders.

They may also have political implications inasmuch as they strengthen opposition to the war, particularly in Britain where Prime Minister Tony Blair is under pressure to justify any loss of life.

So far 18 Britons have been killed by friendly fire or in accidents in Iraq. Fourteen perished in two helicopter accidents last week, and two airmen died when their Tornado bomber was hit by a US Patriot missile on the weekend. Another two British soldiers were killed and two seriously injured when the Challenger II tank they were operating was hit by another Challenger during multiple engagements with Iraqi forces near Basra.

“Friendly fire” incidents have increased steadily since World War II, when, according to the American War Library’s Friendly-Fire Notebook website, 21 percent of fatal and nonfatal casualties were attributed to friendly fire. The site, which referred to US Defense Department statistics, collects and analyzes data on such incidents.

Friendly fire accounted for 18 percent of fatal and nonfatal casualties in the Korean War, 39 percent in Vietnam and 49 percent in the 1990-1991 Gulf War. In last year’s Afghan campaign, US fighter jets mistakenly bombed and killed four Canadian soldiers, dropping a 230-kilo bomb on their unit in a live-fire exercise in April. The two pilots have been charged with manslaughter and assault but a US investigating officer has said they should face administrative punishment rather than a court-martial.

But technology and human error are not the only factors that can create situations where soldiers mistakenly attack their allies. For military powers such as the United States and Britain there is an added complication — the power of the very technology that is supposed to limit friendly casualties and maximize damage to the enemy.

Since in any conflict the forces of both countries are likely to be vastly superior to their adversaries, US and British troops will inevitably be responsible for most of the action — thereby increasing the risk of catastrophic error.

Meanwhile, Britain said yesterday is was almost certain that two dead soldiers whose bodies were shown on Qatar-based Al-Jazeera television were indeed British, as the station claimed. “We have yet to undertake formal identification, but it is probable that these are the two personnel currently listed as missing,” the Defense Ministry said in a statement in London. “The next of kin have been informed that these soldiers have now been categorized as missing believed killed. Our thoughts are with their families and friends.”