He is due to meet with Afghan President Hamid Karzai, US troops and allied commanders during his two-day visit.

Later this month, Mr Karzai will announce the schedule for the handover of security responsibilities from foreign forces to the Afghans.

The visit comes at a time of widespread anger towards the United States over civilian casualties.

On March 6, President Karzai told Gen David Petraeus, the US commander of foreign troops in Afghanistan, that his apology for the deaths of nine children in a Nato air strike last week was "not enough".

The president added that civilian deaths were the main cause of the worsening relationship between Afghanistan and the US.

US President Barack Obama has also apologised for the killings, which he described as a "tragic accident".

The boys, who had been gathering firewood, were mistaken for insurgents and killed by US helicopter gunships.

Hundreds of Afghans rallied to denounce the killings. The protesters condemned both Nato and the Taliban for killing civilians.

The war is now in its 10th year, civilian casualties are at an all-time high, and the population has grown weary of the fighting.

Insurgents are to blame for most of the deaths, but killings by foreign troops generate widespread outrage, our correspondent adds.