Afghanistan president Mr Hamid Karzai has called in US soldiers as protection due to rising security fears following the murder of an Afghan vice-president.
About 50 guards who were replaced were part of the 10,000-strong force of fighters loyal to Defence Minister Mr Mohammed Fahim.
Sources said the first of about 50 US soldiers, including special forces, started deploying over the weekend to take care of Mr Karzai's safety.
Diplomats said the move followed "serious threats" against Mr Karzai. Some of these are believed to have come from within his own cabinet.
US Defense Secretary Mr Donald Rumsfeld confirmed the move, calling it a "short-term" measure to ensure stability in Afghanistan. He said the United States also would help train Afghan bodyguards for Mr Karzai.
Concern over Mr Karzai's safety has soared after the unsolved killing this month of Vice President Abdul Qadir, said a presidential spokesman.
"The investigation into the slaying of Qadir has so far not produced results, and something had to be done to increase the president's security," the spokesman said.
AP