The World Health Organization has warned that wearing masks alone will not be enough to defeat the global Covid-19 pandemic, which has now left more than 70,000 people.

Speaking during a briefing this afternoon, WHO director-general Tedros Adhanom Ghebreyesus also warned that there is no "silver bullet" through which to beat the coronavirus.

"Masks should only ever be used as part of a comprehensive package of interventions. There is no black or white answer, and no silver bullet," he said.

"Masks alone cannot stop the Covid-19 pandemic," he added.

The WHO said that calls for the public to wear masks in the fight against the virus can be justified when other measures like hand-washing and physical distancing are difficult.

Dr Tedros said: "Countries could consider using masks in communities where other measures such as cleaning hands and physical distancing are harder to achieve because of lack of water or cramped living conditions."

Dr Tedros also angrily slammed recent comments made by scientists suggesting a vaccine for the coronavirus should be tested in Africa as "racist".

He described the idea as a hangover from the "colonial mentality".

"Africa cannot and will not be a testing ground for any vaccine," he said, "we will follow all the rules to test any vaccine or therapeutics all over the world... whether it is in Europe, Africa or wherever."

Meanwhile Canada has said it is pressing Washington over a complaint that US officials had blocked the export of three million face masks bought by the province of Ontario.

Ontario Premier Doug Ford told Global News the incident had happened over the weekend and described it as "absolutely unacceptable". Ford said Ontario, the most populous of Canada's 10 provinces, would run out of personal protective equipment in a week.

Separately, Canadian medical health officials said the death toll in the country from Covid-19 had risen to 293 from 258 on Sunday. The number of positive cases is now 15,822.

Allies of the United States are complaining about what they call its "Wild West" tactics in outbidding or blocking shipments to buyers who have already signed deals for medical equipment to fight the coronavirus outbreak.

Canadian Deputy Prime Minister Chrystia Freeland told reporters that Ottawa was aware of "some difficulties" shipping the masks across the border and would do all it could to help.

"We are working intensely with the United States to ensure that all the masks that have been bought can be exported to Canada," she said when asked about Ford's complaint.

"We have had positive conversations and we are continuing to work to settle this matter," she added. The US embassy in Ottawa was not immediately available for comment.

US President Donald Trump has asked manufacturing firm 3M Co to pause its exports of domestically-produced respirator masks to Canada and some Latin American countries.

Canadian Prime Minister Justin Trudeau sidestepped reporters' questions about the issue on Monday, saying his government was in productive talks with the United States and adding: "We expect those shipments to be delivered."