Russia has emerged as the main suspect behind a spate of mysterious illnesses to afflict American diplomats in Cuba and China, according to US officials.

They told NBC News that communications intercepts indicate Moscow was involved but that the evidence was not sufficiently conclusive to publicly blame the Kremlin.

The latest twist comes at a time of strained relations between the two countries. Although President Donald Trump has spoken warmly of Vladimir Putin, his Russian counterpart, his administration has expelled diplomats in response to the poisoning of a former Russian spy in Britain and the US is concerned about attempts to meddle in November's midterm elections.

Americans officials have struggled to work out who or what was behind the health problems first reported at the US embassy in Cuba two years ago.

Victims said they heard a high-pitched whine and later suffered headaches, nausea and dizziness among a range of symptoms that doctors said resembled traumatic brain trauma.