President Volodymyr Zelenskiy noted that an investigation had been launched into the incident and said exerting pressure on journalists was unacceptable.

The video, posted on Tuesday by a little-known online site, whose name translates as "people's truth", purported to show secret recordings of employees of the Bihus.info investigative journalism project.

In separate, recorded phone calls that were also posted online, several people could be heard talking about procuring cannabis and MDMA. A video showed a group of people snorting a powdered substance off a table.

"Such surveillance should be given a legal assessment, regardless of whether or not a possible violation of the law related to the circulation of narcotic substances was made public in the covertly filmed material," the SBU security service said.

Cannabis and MDMA are illegal in Ukraine, although it is in the process of legalising the former for medicinal purposes.

Bihus.info regularly publishes investigative stories about officials and wealthy individuals.

Zelenskiy, in his nightly video address, said the issue of surveillance of journalists' activity had been discussed at a meeting with the SBU and law enforcement officials.

"The SBU has initiated an investigation and will find out all the circumstances," Zelenskiy said. "Any pressure on journalists is unacceptable."

The video follows an incident on Sunday in which another prominent investigative journalist's apartment was visited by unknown men who tried to break down his door.

In a statement on Facebook, Bihus.info acknowledged the video was genuine, condemning both the use of drugs and the surveillance of its employees.

"The use of prohibited substances (even outside of working hours) is a disgrace. And there should be responsibility for this," the statement said.

"But the pressure and persecution of us and our fellow investigative (journalists) is a much bigger scandal," it continued, adding that the video made clear Bihus.info's employees had been under surveillance for a year.

Bihus.info did not directly accuse any person or organisation of responsibility for the surveillance, although editor-in-chief Denys Bihus hinted in a separate video statement that a law enforcement body could be responsible.

Bihus said camera personnel implicated by the recordings had been fired, stressing that none of its reporters was involved.

The SBU said in its statement about the video that it believed the "transparent and unimpeded work of independent and professional media is an important condition for the development of Ukraine as a democratic state".

(Reporting by Max Hunder; Editing by Nick Macfie and David Gregorio)