Afghanistan Rights Monitor (ARM) said Dostum's reappointment as chief of staff to the commander-in-chief, Karzai, was a blow to justice and efforts to start peace talks with the Taliban.
Palace officials confirmed the recent reappointment, which a security source said gives Dostum a largely ceremonial role in charge of the armed forces behind Karzai.
"It is a step ahead in Karzai's old policy of legitimizing prominent warlords and maintaining a state of criminal impunity for them," Ajmal Samadi director of ARM, a nongovernmental group funded by domestic rights campaigners, said in a statement.
"Afghanistan cannot achieve viable peace, stability and prosperity under a government with no commitment to justice."
Dostum had previously held the same position until 2008. That year he was put under house arrest by the government following clashes with a rival, and then left for Turkey in an apparent self-imposed exile.
He returned to Afghanistan days before the presidential election last year, urging his supporters to back Karzai.
Diplomats said the two men had struck an eleventh-hour deal, with Karzai pledging government positions to Dostum's allies in return for his support. Dostum denied any deal.
In January, a member of Dostum's Jumbesh-i-Milli party said they had collected 700,000 votes for Karzai's presidential bid and had been promised several cabinet posts in return.
Both the United States and United Nations expressed concern over Dostum's return. The United States and other countries have accused Dostum of human rights abuses and a U.S. official said in August he may be responsible for "massive war crimes."
Dostum is a leader of Afghanistan's ethnic Uzbek community. He is a former Communist general who led militias through decades of civil war, before joining a loose, U.S.-backed alliance that ousted the Taliban from power in late 2001.
Some 2,000 Taliban fighters who surrendered to Dostum suffocated to death in cargo containers in which they were being held in what became known as the Dasht-i-Laili massacre.
Another 300 Taliban prisoners held by Dostum and U.S. forces in a 19th-century prison fortress died during a rebellion. U.S. President Barack Obama instructed his national security team in July to investigate the alleged mass killing of war prisoners.
Dostum has denied accusations of human rights abuses, including responsibility for Taliban deaths in Dasht-i-Laili.