A reporter and a police officer were killed, said a journalist at the scene who asked not to be named.
The attack comes as armed gangs gain territory in Haiti in the face of under-resourced security forces and little international response, and follows several mass killings in the capital and surrounding rural areas.
Haiti's transitional presidential council said in a post on Facebook that the act would not go without consequences.
"We express our sympathy to all the victims' families, in particular, to the Haitian National Police and all the journalists' associations," it said, without confirming the number of people killed.
Journalists were invited to arrive at the hospital from 8 am in the downtown area of the capital Port-au-Prince to hear from Haiti's new health minister. They were still awaiting the minister when shooting began around 11 am
Health Minister Duckenson Lorthe Blema was installed in November in a cabinet reshuffle when former Prime Minister Garry Conille was ousted after just six months in office.
The State University of Haiti Hospital, known locally as the General Hospital, is the country's largest public hospital but has been closed since a March surge in gang attacks that saw former Prime Minister Ariel Henry ousted from power.
In July, authorities celebrated retaking control of the hospital with a press conference in the facility, but Conille was forced to flee alongside security officers and journalists under a flurry of gunfire.
The hospital is a stone's throw from the central Champ de Mars square, an area that has seen frequent gun battles and clashes between police and a city-wide alliance of gangs known as Viv Ansanm.
Gangs are estimated to control some 80 per cent to 90 per cent of the capital, while neighbouring countries have been slow to deliver on promises of security support for the Caribbean nation.
An international mission approved last year has so far seen just a fraction of troops deploying, while Haitian calls to shore up its resources by converting it to a peacekeeping force met opposition at the UN Security Council.
A spokesperson for the mission, led by Kenya, told Reuters after shooting began that its personnel had not been invited to the conference and it had sent in reinforcements.
Only 24 per cent of health facilities in the Port-au-Prince metropolitan area are currently operating, according to a recent UN report.
