10 June 2014
Last updated at 20:46
All Nato combat forces are due to leave Afghanistan by the end of the year
US military officials are investigating the reported friendly-fire incident in southern Afghanistan that killed five American soldiers and two Afghans.
Rear Adm John Kirby said the US had “reason to suspect that friendly fire is the cause here, specifically friendly fire from the air”.
Adm Kirby said the Pentagon “would let investigators do their work”.
Afghan officials say coalition forces had called for air support to fend off a Taliban attack in Zabul province.
An Afghan soldier and an interpreter were killed in the incident.
On Tuesday, White House press secretary Josh Earnest said President Barack Obama had been informed of the deaths and that his thoughts and prayers were with the families of those killed.
“Our hearts here at the White House are heavy as a result of this loss,” Mr Earnest said.
The incident is among the most serious cases of so-called “friendly fire” in Afghanistan, US military sources confirmed on Tuesday.
Nato-led troops have been battling Taliban and other insurgents in the country since 2001. Militants have stepped up attacks as foreign combat troops leave this year.
Analysis – Military historian Peter Caddick-Adams
Chroniclers of the battle of Waterloo, fought in 1815, have recorded how British infantry squares engaged one another by mistake and other allied cavalry, causing many casualties.
Similar incidents happened in the Crimea in 1854, during the American Civil War of the 1860s and the Boer War of 1899-1902.
War diaries from World War One are peppered with accounts, mainly of British artillery shelling British troops by accident, poison gas clouds being misdirected, or a worn gun barrel firing shells inaccurately.
In World War Two, many allied aircraft were lost to so-called friendly fire, because of poor aircraft recognition skills, or the split seconds in which a pilot had to decide whether to engage an oncoming plane or not.
Historians now think that the famous RAF fighter ace Douglas Bader was shot down by one of his wingmen, not the Germans.
US defence officials told the Associated Press news agency the Americans killed were special operations forces.
Those elite troops are responsible for calling in air support. Under constraints imposed by President Hamid Karzai, they may only do so when they fear they are about to be killed, after concerns over civilian deaths.
The Isaf force currently has soldiers from 50 contributing nations in Afghanistan. Most troops stationed in the south are American.
The incident happened in Arghandab district, a place hotly contested between the Taliban and international forces for some years, BBC’s David Loyn in Kabul reports.
There have been more than 30 Nato forces killed this year in Afghanistan – the latest incident is the deadliest so far in 2014.
Afghanistan is set to hold a run-off round of voting in its presidential election on Saturday.
Insurgents – who vowed to disrupt campaigning and voting – attacked the campaign rally of one of the contenders last week.