At least 30 people are feared to have been killed in a stampede at a popular tourist site in Haiti on Saturday.

The incident took place on Saturday during an annual Easter gathering at the Laferrière Citadel, a Unesco World Heritage site, Jean Henri Petit, head of civil protection for Haiti's Nord department, said, warning the death toll could yet rise.

Prime Minister Alix Didier Fils-Aimé said the incident occurred "during a tourist event attended by many young people" at the site, in the northern town of Milot.

He said an investigation had been launched and "all relevant authorities" mobilised to support those affected.

"The government sends its sincere condolences to the affected families," Fils-Aimé said in a statement.

The site had been packed with students and visitors after the event - commemorating the 19th Century fortress's founding - was advertised on social media, local media report, citing officials.

The stampede was said to have started near the entrance to the site and was exacerbated by the onset of heavy rain.

Haiti's Le Nouvelliste newspaper first reported the death toll, citing Petit. The government statement did not say how many had died.

Citadelle Laferrière, also known as Citadelle Henry, was built by revolutionary Henri Christophe shortly after Haiti gained independence from France.

The stronghold took more than a decade to construct and was a crucial part of a network of fortifications to protect the new Caribbean island nation against attacks. The site has since become a symbol of Haitian independence.

The deadly stampede comes as Haiti grapples with widespread gang violence that has led to thousands being killed.