The US says that 3,200 troops of the 173rd Airborne Brigade will be deployed to Afghanistan in the spring, instead of Iraq.
The US is expecting Taleban attacks in the spring
The reinforcements, currently stationed in Italy, will replace troops from the 10th Mountain Division Brigade who are due to leave Afghanistan in May.
It means that US troop levels in Afghanistan will stay at 27,000, the highest since the invasion of 2001.
The 173rd brigade is expected to stay in Afghanistan for up to a year.
The BBC's Alastair Leithead in Kabul says the brigade's deployment represents a long-term increase in force strength.
Canada says some Nato countries need to show more commitment
Earlier this month an American general took command of Nato forces in the country with the promise of more troops and a pledge for $10.8bn from the US over two years.
The US says that the bulk of the funds will be used to provide resources for the Afghan security forces.
Our correspondent says that developing a strong military to secure Afghanistan will ultimately provide the US and Nato with an exit strategy.
The UK is sending more troops to Afghanistan. The US, UK and Canada have criticised other Nato nations for not committing extra soldiers or resources to the campaign.
On Tuesday, a Canadian Senate committee said the government should consider withdrawing from Afghanistan unless its Nato allies provide additional troops.
It is widely predicted that Taleban insurgents will increase their attacks over the coming months in an effort to pressure the international presence in the country.
Our correspondent says that the huge increase in resources provided by America is being interpreted as an attempt to improve the situation in Afghanistan ahead of presidential elections in the US next year.
He says that the international force still enjoys support from the bulk of Afghan people.