The White House has said that US President Barack Obama does not rule out a complete withdrawal of US troops from Afghanistan by 2014.
President Obama is due to meet Afghan President Hamid Karzai this Friday.
The comments by US Deputy National Security Adviser Ben Rhodes were the first signal that the final decision may be to remove everyone, as happened in Iraq in 2011.
This is despite initial recommendations by the top military commander in Afghanistan to keep as many as 15,000 troops in the country.
Asked about consideration of a so-called zero-option once the NATO combat mission ends at the end of 2014, Mr Rhodes said: "That would be an option that we would consider."
He said the president does not view these negotiations as having a goal of keeping US troops in Afghanistan, adding that the objective was to ensure the training and equipping of Afghan forces and combating Al-Qaeda.
It is expected that it will be months before a final decision is made on troop levels.
In Iraq, Mr Obama decided to pull out all US forces after failing in negotiations with the Iraqi government to secure immunity for any US troops who would remain behind.
The Obama administration is also insisting on immunity for any US troops that remain in Afghanistan, and that question will figure in this week's talks between Mr Obama and Mr Karzai and their aides.
US officials have said privately that the White House had asked for options to be developed for keeping between 3,000 and 9,000 troops in the country, a lower range than was put forward initially by General John Allen, the top US and NATO commander in Afghanistan.
General Allen suggested keeping between 6,000 and 15,000 troops in Afghanistan.
