US commanders have long requested additional forces to fight insurgents
The chairman of the US Joint Chiefs of Staff says up to 30,000 extra troops could be sent to Afghanistan in 2009, almost doubling the US presence.
"Some 20 to 30,000 is the window of overall increase from where we are right now," Adm Mike Mullen said.
The timeframe of deployment has also been shortened, with reinforcements set to arrive by summer at the latest.
On Friday, US Defence Secretary Robert Gates ordered the deployment of a combat aviation brigade by spring.
There are currently 31,000 US troops in the country, 14,000 of whom are part of the 51,000-strong Nato-led International Security Assistance Force (Isaf).
Speaking to reporters after a visit to the Afghan capital, Adm Mullen revealed that the US military planned to as much as double its presence by the middle of next year in order to fight the growing Taleban insurgency.
When we get additional troops here, I think the violence level is going to go up... The fight will be tougher
Adm Mike Mullen, Chairman of the US Joint Chiefs of Staff
"Some 20 to 30,000 is the window of overall increase from where we are right now. I don't have an exact number," he said.
"We've agreed on the requirement and so it's really clear to me we're going to fill that requirement so it's not a matter of if, but when."
"We're looking to get them here in the spring, but certainly by the beginning of summer at the latest," he added.
Earlier statements from the US government and military had suggested about 20,000 troops would be sent, a number which was in line with the minimum requested by the top US and Nato commander in Afghanistan, Gen David McKiernan.
And on Wednesday, Mr Gates said that the US "should think long and hard" before sending significantly more than 20,000 additional troops so they do not become to be seen as an occupying force by Afghans.
Adm Mike Mullen: 'This is a fight... for the future of Afghanistan'
Adm Mullen said most of the reinforcements were likely to be sent to southern Afghanistan to bolster British, Canadian and Dutch troops.
"That's where the toughest fight is," he said. "When we get additional troops here, I think the violence level is going to go up. The fight will be tougher."
The deployment of additional troops was "directly tied" to the gradual withdrawal from Iraq over the course of the next year, he added.
The BBC's defence and security correspondent, Rob Watson, says that the next 12 months are likely to see an increasing focus by the international community on improving governance and development, particularly at a local level, to take advantage off any improved security provided by the extra troops.
Advisers to US President-elect Barack Obama are also stressing the need to look at a regional solution to Afghanistan's problems in the months ahead.
Our correspondent says that although Adm Mullen's announcement hardly comes as a surprise, it is nonetheless a highly significant increase in the US commitment to Afghanistan.
The number of insurgent attacks is increasing and the fighting is spreading
To some extent, the reinforcements represent an acknowledgement by Washington of the country's increasingly fragile security situation in the face of a growing insurgency, he adds.
Already, an additional 3,000 US troops are expected to arrive in January and reports suggest they will be stationed in Wardak and Logar provinces, which have seen fierce fighting in recent months.
The 2,800-strong combat aviation brigade ordered to deploy by Mr Gates earlier this week, which includes Apache attack helicopters as well as Black Hawk and Chinook support aircraft, is expected to move from the US in early spring, officials say.
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