Iran has handed down jail terms of six years and three years respectively to two doctors convicted of plotting to overthrow the government.

A lawyer for the two doctors, brothers Kamiar Alaei and Arash Alaei, announced their sentences Thursday.  He vowed to appeal.  

The two men are well-known for their pioneering work in countering HIV and AIDS.  But Tehran says the men were part of a U.S.-backed plot to create upheaval and undermine the Islamic Republic.

The U.S.-based group Physicians for Human Rights says Tehran convicted the doctors based on what it called "tainted and unreliable" confessions.  A spokeswoman for the organization has said that, far from trying to overthrow any government, Kamiar and Arash were dedicated only to "the battle against HIV/AIDS."

Other human rights groups also have expressed concern about the doctors' case.

Tehran announced last week that it had sentenced two other alleged conspirators with the U.S. to undisclosed jail terms.  Tehran has not identified those two other people. 

Some information for this report was provided by AFP and AP.