Honduran President Juan Orlando Hernandez said on Thursday authorities were investigating an alleged plan by a drug cartel to assassinate him and the US ambassador to the Central American country, James Nealon.

A conservative lawyer who came to power in 2014, Hernandez has reined in homicides with a tough policy against drug cartels that use Honduras as a base to move cocaine into the United States, amid concerns expressed by rights groups.

"They reported there was evidence of plans for an attack against Ambassador Nealon and against (myself) by an Atlantic criminal group," Hernandez told a radio station, referring to a cartel based in the eastern part of the country.

"So we told everyone that they had to do their job, investigate and bring proceedings in the case."

External sources had revealed the assassination plot, Hernandez added, but declined further comment, citing the sensitivity of the case.

It was not immediately clear what role the cartel played in the plan.

The revelation comes a day after Honduran army captain Santos Orellana, under investigation by the United States for alleged corruption and narco-trafficking ties, accused the Drug Enforcement Agency of pressuring him to testify against the president's brother over a plot to kill the US ambassador.

In a letter to Congress on Thursday, the president's brother, Juan Antonio Hernandez, denied any link to illegal activities, and said he was willing to cooperate with any investigation.

Human rights groups have warned against possible violations by security forces in Honduras as they crack down on gangs.