Kenyan President Uhuru Kenyatta has imposed a three-week ban on movement in and out of four main coronavirus "infected areas", including the capital, Nairobi, before the usually busy Easter weekend.

In a televised address to the nation on Monday, Kenyatta stopped short of announcing a full lockdown within these areas but warned, "We must be ready to go even further if necessary."

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The order bars entry and exits in four regions of the country most affected by cases of COVID-19, the highly infectious respiratory disease caused by the new coronavirus. In addition to Nairobi, they include the port city of Mombasa and the counties of Kilifi and Kwale.

"The cessation of movement within the Nairobi metropolitan area shall be for an initial containment period of 21 days with effect from 7pm" on Monday, Kenyatta said in the televised address.

"The movement of food supplies and other cargo will continue as normal during the declared containment period through road, railway and air," he added.

Kenya already has a dusk-to-dawn curfew to contain the virus that has infected 158 people in the country, according to the government's latest tally. Four people have died in Kenya from the disease.

The East African nation of 50 million people has 578 intensive care beds.

"We are at war and we must win," Kenyatta said. "We must make a stand here, before COVID-19 starts to spread out of control," he added.

Forty-three of Africa's 54 countries have closed their borders, according to the Africa Centres for Disease Control and Prevention.

More than half of Africa's 54 countries have imposed lockdowns, curfews, travel bans or other restrictions to try to contain the spread of COVID-19. Fifty-one of the continent's countries have reported cases of the disease.