North Korea's concentration camps have "evolved into a mechanism for extorting money from citizens trading in private markets," wrote the Washington Post on Tuesday quoting a report about to be released by U.S. Congress.

The East-West Center, a research organization established by Congress, will release the report which surveys North Korean refugees in China and South Korea about the gulag in North Korea this week. It interviewed 1,346 North Korean refugees in 11 locations across China in 2004-2005, and 300 in South Korea in 2008. Most were in their late 30s and had been farmers or laborers, the daily said.

It reports an "explosive rise in market activity" in recent days in North Korea. Markets reportedly are the only "source of food for a poor population" suffering from a serious food shortage. But the North Korean regime has banned such market activities and arrested those trading there as "economic criminals" and sent them to the camps.

"Authorities appear to have 'extraordinary discretion' in deciding whom to send to labor camps for economic crimes and in deciding when to let them out," it said. Those arrested can apparently bribe their way out. "About two-thirds of those held in low-level labor camps said they were allowed to go home within a month," the Post added.