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.
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.
'Time running out' The report from the Brussels-based group is stark in its prediction that the Western-backed government in Afghanistan could be on course for what it calls a devastating political crisis after 2014.
It states that "steps to ensure a stable transition must begin now to prevent a precipitous slide toward state collapse. Time is running out."
That same year elections are due in Afghanistan and the report predicts that the forthcoming polls will be as fraudulent as the last ones.
"It is a near certainty that under current conditions the 2014 elections will be plagued by massive fraud. Vote-rigging in the south and east, where security continues to deteriorate, is all but guaranteed," the report warns.
The report also says that Afghanistan's President Hamid Karzai - who is not allowed to run again - may be trying to "stack the deck" for a favoured candidate.
The government statement asserted that forthcoming polls would be "free and fair and without any foreign interference."
But the ICG report concludes that failure to act on these issues could indirectly lead to a political impasse that would provide a pretext for the declaration of a state of emergency, which could in turn lead to the collapse of the state.
It made several recommendations to parliament, the election commission and the international community to help assure a peaceful political transition.
Nato combat troops are set to withdraw by the end of 2014, but a central plank of the strategy is that foreign soldiers will serve alongside and train Afghans for many years to come.