Attackers have opened fire on a beach in an Ivory Coast resort town, sending tourists fleeing through hotels.
A spokeswoman for the Foreign Office said officials were "urgently" trying to establish whether any British nationals had been caught up in the incident.
Photos posted to social media apparently taken at the scene showed bodies sprawled on the beach in Grand-Bassam, a popular weekend destination for both Ivorian nationals and foreigners.
Officials did not say immediately how many were killed, while security forces responded as the area was evacuated and residents hid in their homes.
The beach setting of the incident bears a resemblance to the attack in the Tunisian resort of Sousse last June in which 30 Britons were among 38 tourists killed.
A witness said bursts of gunfire were heard in the beach town, which is about 25 miles east of Abidjan, Ivory Coast's commercial centre, in what is the third major attack on a tourist area in a West African country since November.
Josiane Sekongo, 25, who lives across from one of the town's many beachfront hotels, said she ran outside when she heard gunshots and saw people running away from the beach.
An American embassy delegation was in Grand-Bassam on Sunday, but the US Embassy in Abidjan said it is monitoring the situation and it has no evidence that US citizens were targeted.
The US Embassy Abidjan tweeted: "Monitoring the situation in Grand Bassam. No evidence that US citizens were targeted nor confirmed reports of any US citizens as harmed."
Attacks by extremists on hotels frequented by foreigners in two other West African countries, Mali in November and Burkina Faso in January, killed dozens of people and indicated that extremist attacks are spreading from North Africa.
The historic town of Grand-Bassam is a Unesco World Heritage site.
The travel advice for Ivory Coast on the Foreign Office website warns against all but essential travel to certain parts of the country.
It says: "There is a high threat from terrorism. You should be vigilant after recent attacks in Mali and Burkina Faso. Attacks could be indiscriminate, including in places visited by foreigners."
For Grand-Bassam, the website says people should check travel advice before travelling.
The advice was updated on February 9 and was still said to be current on March 13.