NATO officials said on Thursday that it may fund a force of 352,000 Afghan troops until the end of 2018.
That could saddle the US and its military allies with an extra £1.3 billion a year bill while most of them struggle with budget cuts and financial woes.
Nato agreed last May to underwrite an Afghan force of about 230,000 at a cost of about £2.7bn a year after 2014.
It has cost about £4.25bn this year to fund the current Afghan force of 352,000 and the US is providing about £3.7bn of that.
The greater manpower may boost Afghan forces' confidence and signal that Nato will not cut and run on the country, a Nato official claimed.
Nato secretary-general Fogh Rasmussen admitted that the plan was being considered.
He said that Afghanistan can't afford such a large force and so other countries had to cough up.
"It is actually less expensive to finance Afghan security forces than to deploy foreign troops," he said.
Nato defence ministers are discussing progress in the Afghan war and the reduction of troops.
US President Barack Obama has said that he will withdraw 34,000 US troops from Afghanistan by this time next year.
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