Nigeria has received 4-million doses of Moderna's Covid-19 vaccines donated by the United States government, its health minister said on Monday, as the West African country battles a third wave of infections.

Osagie Ehanire said the vaccines, which arrived on Sunday, are undergoing validation by the country's drug regulator. He said the doses will be distributed to the local states once they are certified fit for use.

The US government last week shipped nearly 10-million doses to two of the most populous African countries - Nigeria and South Africa.

"Vaccination in Nigeria should soon begin with the arrival ... of Moderna vaccines, thanks to the United States government," Ehanire told a coronavirus briefing in Abuja.

He said Nigeria would receive over 40-million doses by the end of the year, without providing details.

The primary healthcare agency said last month that Nigeria had exhausted an initial supply of nearly 4-million shots and expects to receive nearly 8-million doses of Covid-19 vaccines by the end of August, including the US government donation.

Nigeria, Africa's most populous nation, has seen a rise in coronavirus cases since mid July. Some 174 315 cases and 2 149 deaths have been recorded since the pandemic began in early 2020, official data shows.

It recently detected the highly contagious Delta variant, with the health minister warning that the country was going through a third wave of the infection.

Resident doctors in Nigerian public hospitals began an indefinite strike on Monday over grievances that include the delayed payment of salaries and allowances, the doctors' union said, as coronavirus infections rise.