The bus filled with Argentine border patrol officers was traveling in the northern province of Salta when it crashed into a ravine.
Rescue workers rushed to the scene.
So far, officials say about 30 officers are known to have been killed. Many others remain trapped.
The country's newly elected President Mauricio Macri says it's a tragedy.
(SOUNDBITE) (Spanish) PRESIDENT MAURICIO MACRI SAYING:
"I would like to send my most sincere condolences to the their families and the force. We are working on the matter. This has been a very unfortunate tragedy so the Argentine people should be offering their condolences to these families."
Macri suggested the state of the road may have contributed to the accident although an investigation into what caused the crash is underway.