A witness in the resort town of Grand-Bassam told AFP they heard one of the assailants shouting "Allahu Akbar" – Arabic for "God is greatest".
Fourteen civilians and two special forces troops were killed in the shooting spree targeting three hotels, Ivory Coast's President Alassane Ouattara said.
One French and one German national were among those killed, according to Interior Minister Hamed Bakayoko.
SITE Intelligence Group, a US monitor, said Al-Qaeda's North African affiliate had claimed responsibility for the attack in the former French colonial capital, a resort popular with Western expatriates.
Al-Qaeda in the Islamic Maghreb (AQIM) in a statement said three of its fighters had been killed, while Ouattara said six assailants were dead following what he condemned as a "terrorist" attack.
"The toll is heavy," the Ivorian leader said as he arrived in Grand-Bassam, which lies on the Gulf of Guinea around 40 kilometres (25 miles) east of the commercial hub Abidjan.
French President Francois Hollande condemned the "cowardly attack" and offered support for the Ivorian government in finding the perpetrators.
The Paris prosecutor's office said it had opened a terror probe into the attack.
The assailants, who were "heavily armed and wearing balaclavas, fired at guests at the L'Etoile du Sud, a large hotel which was full of expats in the current heatwave," a witness told AFP.
Marie-Claire Yapi, in tears as she held her two-year-old son in her arms, said she was waiting for news of her son and her sister.
"We had just arrived when we heard gunshots – we thought it was a robbery. Someone said to me, 'Run, this is serious, they are killing everyone."
West African nations have scrambled to boost security in the wake of terrorist attacks in recent months in the capitals of Mali and Burkina Faso – assaults on hotels that were also claimed by AQIM.
Sunday's attack also bore grim similarities to the terrorist gun and grenade assault on a Tunisian beach resort last June, which left 37 foreign holidaymakers dead.
'Roamed the beach firing shots'
Witness Braman Kinda said several attackers had "roamed the beach firing shots", while Abbas El-Roz, a Lebanese national who was staying at the Etoile du Sud, said one of the assailants had a Kalashnikov assault rifle and a grenade belt.
Another witness, Kouamena Kakou Bertin, said three of the attackers fled on foot via a nearby road.
A large crowd gathered near the French quarter at the edge of the old town, whose elegant colonial-era facades have earned Grand-Bassam UNESCO World Heritage status.
An AFP journalist saw around a dozen people, including an injured Western woman, being evacuated in a military truck, while the army was tightly controlling access to the area.
In Paris, prosecutors opened an inquiry into the attack, while France's embassy in Ivory Coast told its nationals to stay away from the attack scene "to avoid obstructing security forces".
French Foreign Minister Jean-Marc Ayrault and Interior Minister Bernard Cazeneuve will travel to Abidjan on Tuesday in a show of solidarity with the country, an official in Paris said.
Terrorist threat
The attack is unprecedented in Ivory Coast, the world's top cocoa producer, and comes as analysts voice fears that terrorism could be spreading further into West Africa following AQIM's recent assaults in Mali and Burkina Faso.
The recently-concluded Flintlock military exercise, grouping African, US and European troops, focused on the need to counter terrorism in the region.
Gunmen killed 30 people in Burkina Faso's capital Ouagadougou in January, while the Mali attack the previous November left 20 dead. AQIM claimed both attacks, which both targeted hotels popular with foreigners.
Sunday's attack comes as a blow to Ivory Coast's tourism sector, which the government is seeking to revive as the country emerges from a decade of political crisis.
Former president Laurent Gbagbo is currently on trial at the International Criminal Court for crimes against humanity over deadly violence that followed the disputed 2010 election that brought Ouattara to power.
More than 3,000 people were killed in five months of unrest after the presidential polls, when Gbagbo refused to concede defeat.
Ouattara was re-elected for a second presidential term late last year, hoping to turn the page on the violence and revive Ivory Coast's conflict-scarred economy.
