The General Medical Council (GMC) is monitoring the training of doctors at the Regional Cardiothoracic Surgery Department in Belfast.
It follows concerns about training being raised with the Northern Ireland Medical and Dental Training Agency.
The GMC has implemented “enhanced monitoring” in which it helps bodies to address concerns and find solutions.
The department is part of the Belfast Health Trust, which said all training was regularly reviewed.
The unit provides surgical treatment of the heart, lungs and other chest cavity organs for the whole of Northern Ireland.
It is based at the Royal Victoria Hospital in Belfast, with some surgeries taking place in the Belfast City Hospital.
The Northern Ireland Medical and Dental Training Agency, which oversees postgraduate medical and dental education and training, was told of concerns about the training at the department.
The GMC has subsequently told BBC News NI said it was “concerned about the training environment in cardiothoracic surgery”.
Professor Colin Melville, the GMC’s medical director and director for education and standards, said the enhanced monitoring had been put in place in order to “drive positive change”.
“We’ll keep working closely with the Belfast Trust and the Northern Ireland Medical and Dental Training Agency to ensure a sustainable solution is found at the earliest opportunity for trainees.”
The Belfast Trust said it works with the Northern Ireland Medical and Dental Training Agency to review all training in the trust area on a regular basis.
“All concerns relating to training are considered carefully,” it added.
“The trust takes active steps to address any concerns in a timely manner, including seeking advice and support from both the Northern Ireland Medical and Dental Training Agency and the General Medical Council.”