The United States on Thursday imposed sanctions on Rafael Ortega Murillo, the son of Nicaraguan President Daniel Ortega, alleging he is guilty of corruption and money laundering to enrich his family.

"Rafael Ortega is the key money manager behind the Ortega family's illicit financial schemes," US Treasury Secretary Steven Mnuchin said in a statement.

"Treasury is targeting Rafael and the companies he owns and uses to launder money to prop up the Ortega regime at the expense of the Nicaraguan people."

Rafael Ortega works with Rosario Maria Murillo De Ortega, the vice president and first lady, "to generate profits, launder money and gain preferential access to markets," the US statement said.

The US has already slapped sanctions on the president, his wife and Laureano Ortega Murillo, another of his sons.

Ortega, in power in Nicaragua since 2007, was most recently elected in 2016 for a mandate that would keep him in office until 2021.

Regime forces and pro-government militias have been blamed for more than 300 deaths since April last year, when protests against his rule mushroomed into an uprising that was brutally suppressed.

Critics accuse Ortega, a former rebel hero who was first elected in 1984, of running a repressive dictatorship.

The Organization of American States released a report in November saying Ortega's government "controls and subordinates" branches of government so tightly that it "makes the democratic functioning of the country unviable."

The government has attributed the protests to a failed coup attempt, and Ortega, 74, has accused Catholic bishops of being "coup plotters" for supporting the protesters.

Last month, opposition protesters held a hunger strike inside the Catholic cathedral in the capital Managua, demanding the release of political prisoners.

The EU has also threatened sanctions on Nicaragua over rights abuses and called on the government to resume talks with the opposition.