An Oxfam report last week said a "major change of direction" was needed to prevent a humanitarian disaster and a US study warned Afghanistan could become a "failed state".

Mr Alexander told BBC1's Andrew Marr Show that improving conditions in the war-torn country would be a "long-term struggle and urged Nato nations who have been reluctant to deploy troops in the most dangerous parts of Afghanistan to take a more active role.

US Secretary of State Condoleezza Rice - who will have a meeting with Mr Alexander when she visits the UK next week - has said the alliance needs to take a "hard look" at what more it can do in Afghanistan.

Germany and France are among the nations which have been criticised for failing to send forces to areas of Afghanistan which have seen the most intense fighting.

Mr Alexander said: "There are about 38 Nato countries represented in the ISAF mission. We have made clear to our Nato partners that we do want to see appropriate burden sharing, not just in the number of troops on the ground but where those troops are committed within Afghanistan."

The issue will be raised by Defence Secretary Des Browne in a summit with Nato defence ministers next week.

Mr Alexander said reports that Afghanistan's president Hamid Karzai criticised the British military effort were "not an accurate reflection" of what he said.

"Hamid Karzai is the democratically elected president of Afghanistan. The fact he is democratically elected shows that, not withstanding all of the real challenges - poverty, narcotics, insurgency - we are making progress in Afghanistan.

"It's important that we continue to support the democratically-elected government of Afghanistan."