The assailants fired on beach-goers in the town of Grand Bassam, about 40km (25 miles) east of the country’s commercial capital Abidjan.
The resort is popular with local people and foreigners. Security forces have been sent to the scene.
Ivory Coast was seen as a model of stability in West Africa until a civil war erupted in 2002.
The conflict pitted the mainly Muslim north against the largely Christian south. Since then, peace deals have alternated with renewed violence.
AFP news agency quotes a military official as saying five people were killed, but other reports put the death toll higher.
A witness of Sunday’s attack told AFP news agency that “heavily armed men wearing balaclavas” had opened fire near the L’Etoile du Sud hotel, which was full of expats in the current heatwave.
Another eyewitness, Souleymane Kamagate, says he saw people running from the beach and fleeing in all directions.
BBC regional reporter Maud Jullien says Ivory Coast has been identified as one of several countries in West Africa at risk of being targeted by Islamist militants.
Luxury hotels were targeted by such groups in Mali in November and Burkina Faso in January.