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The World Health Organization voiced alarm Monday over a rapid worsening of the COVID-19 situations in Brazil and Mexico, urging them to be "very serious" about halting the spread.

Both countries had seen both cases and deaths from the novel coronavirus surge in recent weeks as a second wave of the pandemic has hit.

"I think Brazil has to be very, very serious," WHO chief Tedros Adhanom Ghebreyesus told reporters, warning the situation there was "very, very worrisome".

And he echoed the same concern when asked about Mexico, saying that country was "in bad shape".

"The number of cases doubled and the number of deaths doubled... we would like to ask Mexico to be very serious."

Brazil has been one of the countries hit hardest by the pandemic, with more than 172,000 people killed—the second-highest number in the world, following the United States.

After a seemingly endless plateau, with more than 1,000 deaths a day from June to August, on a seven-day rolling average, the numbers had finally been falling in the giant nation of 212 million people.

But Tedros pointed out that while the first week of November had seen 2,538 deaths, last week's death toll in Brazil stood at 3,876 —"a significant increase".

Case numbers had also effectively doubled over the same period, with Brazil facing 218,000 cases last week alone.

President Jair Bolsonaro, who has downplayed the pandemic since the outset, has also dismissed talk of a second wave as "gossip".

The 65-year-old far-right leader, who has himself had COVID-19, argues the economic impact of lockdown measures are worse than the virus itself, and has also said he would not take a vaccine when one becomes available.

Mexico meanwhile saw its total death toll pass 100,000 on November 20 and has added more than five thousand deaths since then.

Over the weekend, for the first time, it counted more than 12,000 cases in a single day.