Kabul, Afghanistan (CNN) -- Five British soldiers were killed and six others were wounded when an Afghan National Police officer opened fire on them, military officials said Wednesday.

The attack happened Tuesday afternoon in the Nad-e'Ali District of Afghanistan's southern Helmand province, Britain's Ministry of Defence said Wednesday. All the soldiers died of gunshot wounds sustained in the attack.

Initial reports show that the Afghan policeman initiated the fire on the soldiers, a ministry spokesman told CNN.

The soldiers had been living and working at an Afghan National Police checkpoint just outside Nad-e'Ali for the past two weeks, mentoring the policemen there, said Lt. Col. David Wakefield, a spokesman for Task Force Helmand.

An initial assessment of the attack shows that "an individual Afghan National policeman, possibly acting in conjunction with ... another, started firing inside the checkpoint before fleeing the scene," Wakefield said.

Top 10 ISAF troop contributing nations
United States: 65,000
United Kingdom: 9,000
Germany: 4,245
Canada: 2,830
Italy: 2,795
Netherlands: 2,160
Poland: 2,025
Australia: 1,200
Spain: 1,000

Source: ISAF A joint team from NATO's International Security Assistance Force and the Afghan Ministry of Interior were investigating the incident, ISAF said.

If the reports are true, it would not indicate a wider problem, said Abdullah Abdullah, an Afghan politician who was the chief challenger to President Hamid Karzai before pulling out of the recent election.

"I don't think that this is the example of a broader picture, but this could be an example of corruption ... or somebody might have been paid, somebody might have had connections," former Afghan presidential candidate Abdullah Abdullah told Sky News about the reports.

"These are the things you cannot take care of (individually). This has to be taken care of by those who are in charge of the police force," he said.

Afghan Interior Minister Haneef Atmar called it a "senseless attack" and said it appeared to be an isolated incident.

Gen. Stanley McChrystal, the ISAF commander, said Atmar promised the incident would be fully investigated.

"We will not let this event deter our resolve to building a partnership with the Afghan National Security Forces to provide for Afghanistan's future," McChrystal said in a statement.

Three of the dead soldiers were from the Grenadier Guards and two were from the Royal Military Police, Britain's Ministry of Defence said. Their next of kin have been informed.

Two Afghan National Police officers were wounded in the attack, ISAF said.

Wakefield expressed his "deepest sadness" at the deaths.

"Five British soldiers, five of our own, shot down in the course of their duty. They will not be forgotten," Wakefield said.

British Defense Secretary Bob Ainsworth also expressed his sadness and condolences, as did British Prime Minister Gordon Brown.

"The death of five brave soldiers in a single incident is a terrible loss," Brown said in a statement. "My thoughts, condolences and sympathies go to their families, loved ones and colleagues. I know that the whole country too will mourn their loss."