President Donald Trump had previously warned that he would be cutting the size of US forces in the two countries.
The cuts will take effect before President Trump leaves office, the Pentagon said.
The Secretary-General of the Nato alliance, Jens Stoltenberg, has warned of a "high price" if US and allied forces leave Afghanistan too quickly.
In a statement, he said the country risked once again becoming a platform for international militants to organise attacks.
