US Secretary of State Condoleezza Rice and British Foreign Secretary David Miliband have arrived in Afghanistan in a symbolic show of unity, pressing the case for reluctant allies to share the combat burden.

The visit comes shortly after US Defence Secretary Robert Gates warned that the future of the North Atlantic Treaty Organisation (NATO) military alliance is at risk.

Dr Rice said NATO members needed to "come together to give enough military power to do what needs to be done on the front end of the counter-insurgency effort".

"Frankly, I hope there will be more troop contributions and there need to be more Afghan forces," Dr Rice said on the flight from London.

Earlier, Mr Gates expressed his concern on the eve of a NATO defence ministers' meeting in Lithuania.

He said differences over Afghanistan mean it could become a two-tier alliance.