A new assessment of Afghanistan's future says the country could once again become a terrorist haven unless U.S. and international partners underwrite a much larger Afghan security force than is currently planned beyond 2014.

The study released Thursday also concludes that this larger force and the government ministries to support it will require international trainers and advisers at least through 2018. The Obama administration has not yet decided whether to play that role beyond 2014.

The study was ordered by the Congress and conducted by CNA Strategic Studies, a federally funded research group.

It concludes that military pressure from the Taliban is likely to increase in coming years. That contradicts an underlying assumption of NATO's decision to sharply shrink Afghan security forces after 2014.