Around half a dozen police officers and support staff are under investigation in Northern Ireland over allegations that they posted racist, sexist and sectarian messages on Twitter and potentially prejudiced criminal inquiries, the Guardian has learned.

The group is also being investigated for allegedly tweeting pictures of bottles of urine from inside a police vehicle and showing off their private parts beside guns.

The Police Service of Northern Ireland confirmed that an inquiry is under way in relation to the tweets, which have been running on the social media network for a year.

PSNI assistant chief constable Mark Hamilton, who is in charge of the force’s professional standards department, told the Guardian: “I can confirm that PSNI is examining material posted on personal social media accounts by persons who may be serving police officers.

“In response PSNI professional standards department has instigated disciplinary proceedings against one individual who has been identified as a serving officer. The review of the material is ongoing at this stage.”

It is understood the six PSNI officers or civilian support staff under investigation are all based in the greater Belfast area.

The tweets include wholly unfounded fake rumours of a sex tape involving a Northern Irish journalist with references to oral sex taking place in a doorway.

Another journalist from a local Belfast radio station is referred to as a “cunt” in a different tweet.

In another tweet, one of those alleged to have sent the messages brands the Catholic Lower Falls-Divis district of West Belfast as a place where “thieves operate... like fucking rabbits”. Beneath the message is a map of the district with a red circle drawn around it.

Another post relating to the same district includes a photograph of a man alleged to have mugged a foreign tourist earlier this year.

Security sources in Northern Ireland said that this tweet was sent out before any arrest was made in connection with the crime. In tweeting the suspect’s face, those responsible were in danger of prejudicing any upcoming trial in connection with the mugging, the sources said.

There is also concern in security circles that those allegedly engaging with women online for sexual encounters could fall for “honey traps” set by armed republican dissident groups determined to kill PSNI members.

Some of those behind the messages even engage in Twitter exchanges with Saoradh – the new political party that supports prisoners belonging to the New IRA terror group.

The republican dissidents are taunted and told that “my colleagues sit in on your meetings & private chats anyway. They fill me in.”

The tweet end with the hashtag #C3, which is a reference to the PSNI’s anti-terrorist branch.

A security force veteran based in Northern Ireland, who brought the tweets to the attention of the Guardian, said those behind this particular message had openly admitted to the republican dissidents that they are being monitored via electronic surveillance.

Pictures of the interiors of heavily-fortified PSNI stations, including a CCTV monitoring station in Belfast, have also been tweeted in the stream being investigated. Other pictures of alleged crime suspects and convicted criminals sent include a message urging a prisoner to “put the rope up” and “hang yourself”.

One security source said: “It’s wrong of me to use the term officer when talking about these individuals with multiple personalities, it’s an insult to the good and honourable officers who serve the community of Northern Ireland in a professional manner.”

The organisation representing rank and file PSNI officers said the ongoing inquiry was a lesson for its 6,500-strong membership to be extremely cautious about using social media networks.

Mark Lindsay, chair of the Police Federation for Northern Ireland, said: “We are fully aware of the advantages of using Twitter to communicate with others and, indeed, the wider community. It is a public forum, and we would urge our officers to exercise great care and caution in their use of all social media platforms.

“There are dangers and pitfalls. There is also still a ‘severe’ threat to our officers and it’s essential for their own safety, and the safety of their colleagues, that officers realise the risks and take all necessary precautions to avoid unnecessary attention.”