WASHINGTON (AP) -- A new assessment of Afghanistan's future says the country could revert to a terrorist haven unless U.S. and international partners underwrite a larger - and more expensive - 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. U.S. military commanders have recommended such a role following the withdrawal of all U.S. and NATO combat troops in December, but the Obama administration has not yet committed to it.
The study was ordered by the Congress and conducted by CNA Strategic Studies, a federally funded research group.
It describes in detail what is at stake for the U.S. at an important juncture of the war, which was launched by President George W. Bush in response to the 9/11 attacks orchestrated by al-Qaida, then based in Afghanistan.
President Barack Obama is weighing his options in Afghanistan, aware of the American public's war weariness as well as the risks of failing to ensure that Afghanistan does not once more become a sanctuary for al-Qaida.
The U.S. currently has about 33,600 troops in Afghanistan, down from a high of 100,000 in 2010.
U.S. and coalition combat operations are to end by Dec. 31, but the international military presence beyond that is still in doubt. Obama has said the U.S. might keep some number of troops there for counterterrorism and training missions, but that cannot happen unless the Afghan government signs a security accord that establishes the legal basis for a continued U.S. presence.
President Hamid Karzai negotiated terms of the security deal last year but has refused to sign it. U.S.-Afghan relations have been damaged by a series of other recent actions, including Karzai's decision earlier this month to release 65 prisoners over strenuous objections by the U.S., which deems them to be threats.
A central recommendation of Thursday's report on the outlook for Afghan security beyond 2014 is that planned steep reductions in the size of Afghan army and police forces not be carried out as currently envisioned by NATO.
In deciding at their May 2012 summit meeting to reduce Afghan forces to 228,500 troops, NATO leaders assumed that by 2015 the Taliban would pose a much diminished threat.
"Our threat assessment finds this assumption to be faulty," the report said. It described the Taliban as weakened but still a viable threat to the Afghan government.
The Afghan forces currently stand at about 352,000, plus another 30,000 members of an auxiliary known as the Afghan National Police. Thursday's report recommended a minimum base force of 344,300, plus 29,100 auxiliary forces. It said this might cost roughly $5 billion to $6 billion a year to sustain - nearly all of which would have to be contributed by the international community.
NATO put the price tag of its envisioned smaller force of 228,500 at $4.1 billion a year.
The report sketched a scenario in which the Taliban would seek to capitalize on the absence of foreign combat forces and press its campaign to regain political power in Kabul.
"History suggests that the Taliban will use sanctuaries in Pakistan to regenerate their capabilities as military pressure on the movement declines," the report said.
As a first step, in 2015-16, the Taliban may put additional pressure on Afghan security forces in rural areas, expand their control and influence in areas vacated this year by coalition forces, encircle key cities and conduct high-profile attacks in urban areas, the report said. By 2016-18, after recovering from the military pounding it has taken in recent years from U.S. and coalition forces, the Taliban may be in position to press a more aggressive and intense military campaign - beyond what NATO has assumed is likely.