An Afghan policeman shot dead three British soldiers at a checkpoint in southern Helmand province on Sunday, Afghan officials said, the latest in a chain of increasingly frequent rogue killings.

 

A fourth British soldier was also injured, provincial governor spokesman Daoud Ahmadi said of the attack, Reuters reported on Monday.

 

The soldiers were serving with an Afghan police advisory team and were killed after a meeting at the checkpoint at Nahr-e-Saraj in Helmand, Britain's Defence Ministry said on Monday.

 

The assailant was injured and detained, a Defense Ministry spokesman said.

Defense Secretary Philip Hammond said the deaths, which take to 422 the number of British military personnel killed in Afghanistan since the NATO-led war began in 2001, would not impact Britain's mission there.

 

"Though deeply tragic, yesterday's incident and attacks like it will not derail the mission or distract us from the task in hand," Hammond said in a statement.

 

So-called green on blue shootings, referring to the uniform of the Afghan security forces and the color of NATO, have severely strained ties between President Hamid Karzai and his Western backers.

 

Before Sunday's assault there were 17 such attacks this year and 23 NATO soldier deaths, compared to the 11 attacks and 24 deaths for the same period last year, NATO figures show.

 

But NATO commanders argue the growing number of shootings is in proportion to the growing size of Afghan security forces toward an eventual 352,000 target.

 

After the United States, Britain has the second-largest number of troops in Afghanistan at about 9,500, and is due to withdraw 500 by the end of the year.

 

Two of the soldiers killed on Sunday were from the 1st Battalion Welsh Guards and one was with the Royal Corps of Signals.

 

However, roadside bombs and IEDs are by far the most lethal weapons, the Taliban have used against Afghan forces and occupying foreign troopers over the recent years. 

 

According to the independent website, icasualties.org, as many as 223 foreign troopers, mostly Americans, have lost their lives in Afghanistan so far this year. 

 

A total of 566 U.S.-led forces died in Afghanistan in 2011. However, 2010 remains the deadliest year for foreign military casualties, with a death toll of 711. 

 

The high number of military casualties in Afghanistan has intensified opposition in the United States and other NATO member states to the U.S.-led war in the violence-scarred country. 

 

The war began in 2001. The offensives removed the Taliban from power, but insecurity continues to rise across Afghanistan, despite the presence there of about 130,000 US-led troops.