A British national living in Sierra Leone has tested positive for the Ebola virus – the first Briton confirmed to have contracted the disease.

The Department of Health said medical experts were assessing the situation "to ensure that appropriate care is delivered". Consular assistance is being provided.

Professor John Watson, deputy chief medical officer, said: "The overall risk to the public in the UK continues to be very low. Medical experts are currently assessing the situation in Sierra Leone to ensure that appropriate care is provided.

"We have robust, well-developed and well-tested NHS systems for managing unusual infectious diseases when they arise, supported by a wide range of experts."

Meanwhile, west African nations have imposed stringent new measures to stop the spread of Ebola as two new cases emerged in Nigeria that appear to indicate a widening of the circle of those affected.

The Ivory Coast has closed its borders with its Ebola-hit neighbours and Sierra Leone has said it will jail people who hide patients with the virus. The World Health Organisation (WHO) believes the practice has contributed to a major underestimation of the current outbreak.

It was announced on Saturday that the borders between Ivory Coast and both Guinea and Liberia had been closed the previous day as the death toll of the epidemic reached 1,427 across west Africa.

Agence France-Presse reported that the measure was put in place "to protect all people, including foreigners, living on Ivorian territory," Ivorian prime minister Daniel Kaban Duncan said.

According to the WHO, Liberia has been the hardest hit by the outbreak, with 624 deaths since March.

Guinea has seen 406 people die, while in Sierra Leone 392 have succumbed to the haemorrhagic fever. Nigeria, meanwhile, has seen five people die.

The two new cases in Nigeria are causing particular concern because they are outside the immediate group of caregivers who treated a dying airline passenger.

The new patients were infected by their spouses, both medical workers who had direct contact with Liberian-American Patrick Sawyer, who flew into Nigeria from Liberia and Togo and infected 11 others before he died in July.

There are fears that the number of people affected is even higher than reported because many people – mindful of the stigma attached – are hiding Ebola patients.

"This phenomenon strongly suggests the existence of an invisible caseload of patients who are not being detected by the surveillance system," the UN health agency said. This has "never before been seen in an Ebola outbreak", it added.

According to the Associated Press, the Liberian government has delivered donated rice to a slum where 50,000 people have been sealed off from the rest of the capital Monrovia in an attempt to contain the outbreak.

In Sierra Leone, a new amendment to the country's health act was passed, imposing jail time of up to two years for those caught hiding Ebola patients. The new law – an update to the country's 1960 Public Health Act – was passed on Friday.

Lawmaker Ansumana Jaiah Kaikai said the measure was necessary to compel reluctant residents to cooperate with government officials. "This amendment seeks to address these emerging bottlenecks," he added.