Britons in the Kenyan capital of Nairobi have been urged to remain vigilant in the wake of a deadly terrorist attack on a luxury hotel complex.

At least 15 people are reported to have been killed in the raid by militants on an area which houses the 101-room DusitD2 hotel and offices in broad daylight yesterday afternoon local time.

The attack started with an explosion which set several cars on fire in the hotel car park and was followed by a blast said t ave been set off by a suicide bomber in the hotel lobby. Gunmen then burst in throwing hand grenades and shooting indiscriminately.

By the evening officials said the siege was over but gunfire and explosions were still heard early on Wednesday and a security operation is ongoing, the BBC reported.

Those who are known to have died include a British man and an American woman, according to a Sky News report.

The British High Commission in Nairobi will be closed for routine business today while staff respond to the incident, the Foreign and Commonwealth Office announced.

In an updated travel advisory, the FCO said: “There is an ongoing security incident in the Riverside area of Nairobi; Kenyan authorities are conducting an operation in response to the incident.

“If you’re caught up in the incident, turn any mobile phones or other devices to silent and do not put your location on social media.

“If you’re able to do so safely, leave the area and contact your friends and family to let them know you are safe. Remain vigilant and follow the advice and instructions of local authorities.

“If you’re in the UK and worried about a British national in Kenya, call the FCO in London on 020 7008 1500. This page will be updated as the situation develops.”

In a further update this morning, the FCO said: “The Kenyan government has stated that the security operation in the Riverside area of Nairobi has concluded.”

More than 300 people were rescued from the complex and taken to a nearby trauma centre in the early hours of this morning. About 30 people were being treated in Nairobi hospitals while the security operation is ongoing.

The Somalia-based militant group al-Shabab claimed responsibility for the attack.

Al-Shabab extremists were responsible for an assault on the Westgate Mall shopping centre in Nairobi in 2013, which resulted in a days-long siege that left 67 people dead.