A new report on Afghanistan warns that the departure of Nato forces in 2014 could be followed by the government's collapse and even civil war, unless steps are taken now.
The International Crisis Group (ICG) says the Afghan police and army are unprepared for security responsibility.
It added that polls in two years' time would be "plagued" by fraud unless the state acted to ensure a clean vote.
The Afghan government labelled the predictions "nonsense and garbage".
"Our nation was not born in 2002. We have a history of 5,000 years. We have fought against superpowers in the past. Our national police and army are ready to defend the country's soul and sovereignty," a statement said.
The government spokesman added that if the international community fulfilled its pledges of future support, Nato's withdrawal in 2014 would not make any difference.
Nato Secretary General Anders Fogh Rasmussen said he did not share the ICG's negative assessment.
He told the BBC he was confident the Afghan security forces would be able to take charge after 2014.