A former undercover agent with the Drug Enforcement Administration has pleaded guilty on charges of stealing more than $700,000 in digital currency – most of it from an online drug bazaar known as Silk Road that he infiltrated as part of a federal investigation.

Carl M Force, dressed in the orange garb of the San Francisco jail and shackled at the ankles, pleaded guilty to extortion, money laundering and obstruction of justice in federal court.

The former DEA agent could face a maximum sentence of 20 years in prison when sentenced in October.

"He had a stellar 15-year career with the DEA except for this one blip," Force's attorney, Ivan Bates, said outside court.

Inside court Bates told US District Judge Richard Seeborg that Force suffered from anxiety and depression.