The first transfer of radioactive material from Dounreay to the US as part of an agreement between London and Washington took place at the weekend, a news report said.
The materials were flown from Wick John O’Groats Airport on Saturday aboard a US military Lockheed C-17 transport aircraft, the BBC reported.
In exchange, the UK will receive a type of uranium used to diagnose cancer as part of the pact, the report said.
Highlands and Islands MSP Maree Todd criticised the flight, according to the report, saying it had been conducted in more secrecy than was required.
The transfer deal was announced by former UK Prime Minister David Cameron in February.
The Dounreay nuclear testing facility near Thurso is currently being decommissionined.
A spokesman for Dounreay was cited as saying by the BBC: “All nuclear materials are being removed from the site. This programme started in 2001 and we expect it to continue for a number of years.
“Our priority at all times is to comply with regulations governing the safe and secure transportation of nuclear material, both in storage and transit.
“Compliance with these regulations includes protecting information about routes, dates, timings and locations of nuclear material in any current or future transport.”