Gunmen stormed three hotels Sunday in the beach resort city of Grand-Bassam about 40 kilometers (25 miles) from the country's largest city, Abidjan, officials said. Foreign nationals, including four French citizens and a German woman, were among the dead.
Al Qaeda in the Islamic Maghreb claimed responsibility on a social media account tied to the group.
Three terrorists also were killed in the attack, the Ivory Coast presidency said on its Facebook page Monday, citing Interior Minister Hamed Bakayoko.
On Sunday, Ouattara had given the number of terrorists killed in the attack as six. No information was immediately available Monday clarifying the discrepancy.
Ouattara declared three days of national mourning. He urged Ivorians to come together and to not give in to fear.
"I condemn in the strongest terms these heinous and cowardly attacks on innocent people," he said. "Ivory Coast will not be intimidated by terrorists."
Popular weekend spot targeted
The attack began shortly after midday Sunday when the beach at Grand-Bassam reportedly was packed with people swimming, relaxing and eating at beachside cafes. The area is popular with wealthy Ivorians and Westerners, particularly on weekends.
Security surveillance footage from one bar shows the confusion as shots rang out, causing people to flee. Moments later, a gunman in a waistcoat calmly walks by, firing his AK-47.
Isaac Ouattara, a hotel manager who was there that day, said he saw three men wearing jeans and white shirts shooting as they walked down the beach.
"I was standing on the balcony. I saw everything. They shot a man, then a woman. They were shouting "Allahu akbar!" he said. "One of the gunmen was injured in the leg, so the other two just shot him dead."
Photos posted by Twitter user @lopezdobe showed ammunition magazines and grenades on the ground near the scene.
The United Nations joined the U.S., French, German and other governments in denouncing the attack.
Among the victims was a German woman, Henrike Grohs, 51, who worked as a director of a local German cultural association.
French President Francois Hollande
had earlier said that at least one French citizen was killed in the "cowardly attack." In a statement later Monday, the Elysee Palace said four French nationals were killed. It was not clear from the statement if the number of dead rose or if they are included in the 18 confirmed deaths.
The White House also condemned the attack "in the strongest terms," National Security Council spokesman Ned Price said. The United States and France have pledged their support to the Ivorian government to help find out who was responsible.
The French interior and foreign ministers were scheduled to travel to Abidjan on Tuesday, according to the statement.
Al Qaeda offshoot was behind other hotel attacks
Sunday's violence was the third major attack on hotels in West Africa claimed by al Qaeda in the Islamic Maghreb since November.
Al Qaeda in the Islamic Maghreb, al Qaeda's North African offshoot known as AQIM, has its roots in Algeria. The group has been stepping up attacks
after a downturn in activities.
Ryan Cummings, director at Signal Risk, an Africa-focused risk management company, said the recent attacks were evidence of a resurgence.
"They've executed attacks on hotels before, but none at the level of sophistication or as spectacular as have been in recent months," he said.
He said the chosen targets reflected the group's pronounced "anti-French" focus.
"They're almost ticking off major hotels within Francophone West Africa, specifically hotels that are quite popular with French citizens," he said.
Targeting French interests had been a priority for the group since France launched, at Mali's request, Operation Serval
, a U.N.-sanctioned ground and air operation against jihadists in 2013. It was succeeded in August 2014 by Operation Barkhane, a broader French anti-terror mission targeting Islamists in the Sahel, with a 3,000-strong force headquartered in Chad.
While Serval was successful against Islamist militants in Mali, subsequent operations have been less effective, allowing militants across the region to reconsolidate, he said.
The SITE Intelligence Group, which tracks jihadist organizations, said in an analysis of Sunday's attack that AQIM had "escalated its operations in West Africa, specifically targeting Europeans, as well as other foreigners, in hotels and resorts."
But targeting Westerners isn't the only goal, according to SITE.
"AQIM is also competing with ISIS
for reassertion and dominance in West Africa, among other regions," the group said.