Washington: Russia is the main suspect in US agencies' investigation of mysterious illnesses in American embassy personnel in Cuba and China, NBC News reported on Tuesday.

Evidence from communications intercepts has pointed to Moscow's involvement during the investigation involving the FBI, CIA and other agencies, NBC reported, citing three unidentified US officials and two other people briefed on the probe.

The evidence, however, is not conclusive enough for the United States to assign blame publicly to Moscow, according to the TV network's report.

The FBI said it did not have a comment on the report. A US government source familiar with official assessments said intelligence agencies would not confirm the report to Reuters.

US officials said in July that they are still investigating health problems at the US embassy in Cuba, and do not know who or what was behind the mysterious illnesses, which began in 2016 and have affected 26 Americans.