The letter said the USS was going to be sending a small group of troops to Afghanistan.
They were also going to conduct the deployment of a small number of US drones to Pakistan, but the request was not to deploy a full brigade of troops, it said.
The letter was sent on the same day that Pakistan announced it had deployed more than 2,000 troops to its border with Afghanistan.
A day later, the US deployed a brigade of its own to the country.
The move came amid rising tensions between Islamabad and Washington, the biggest of which is over the contested territory in the South China Sea.
The letter did not say exactly what the troops would be doing, but US officials said they were sending a smaller force to the area, but were not deploying an entire brigade of US troops there.
Read moreThe US has been sending drones to Afghanistan to carry out strikes on militants and other targets, as well as to conduct surveillance and reconnaissance.
Pakistan says its troops are there to help the US fight al Qaeda, a charge Washington has rejected.
But the deployment comes amid growing tensions between Washington and Islamabad over the South East Asia region, where it has accused Islamabad of being a transit country for terrorists fleeing US drone strikes.
This month, the Pakistani government said that an unmanned drone that crashed in Pakistan in April was carrying a large amount of explosives, and was likely to be used in an attack.
US officials have said that while they have no direct evidence of a strike in Pakistan, the presence of US soldiers on Pakistani soil could complicate efforts to prosecute al Qaeda in the Arabian Peninsula (AQAP), which has been linked to several attacks in the region.
Pakistan said the drone was a drone belonging to a US company, but that it had no evidence that it was carrying weapons.
A spokesperson for the Pakistan Defence Ministry said the drones presence in Pakistan was routine and the government did not comment on US military operations in the country, as it does not have a formal relationship with the US.