Mr O'Brien said the United States will look at restrictions for other countries in the Southern Hemisphere on a country-by-country basis.
"Brazil is having some trouble, no question about it".
"We've seen many South American countries with increasing numbers of cases and clearly there's a concern across many of those countries, but certainly the most affected is Brazil at this point", said Mike Ryan, executive director of the W.H.O.'s emergencies program.
He said the United States would look at restrictions for other countries in the Southern Hemisphere on a country-by-country basis.
As of Friday, Brazil reported over 347,000 COVID-19 cases, second only to the United States in the number of infections, according to Johns Hopkins University. Press secretary Kayleigh McEnany said the decision was made "to protect our country".
Bolsonaro's insistence on the drug's potential and defiance of state isolation orders pushed out two health ministers, both doctors, before the current acting head, an army general, issued guidelines to expand use of the drug in coronavirus cases.
O'Brien said on CBS's "Face the Nation" that he hoped any action taken would be temporary - although restrictions placed on other countries haven't been lifted so far.
Brazil's foreign ministry called it a technical decision in the context of "important bilateral collaboration" to fight the Covid-19 pandemic, highlighting USA donations of $6.5 million and a new White House promise of 1,000 respirators. The Trump administration now bars entry of non-citizens traveling from Brazil.
"I don't want people coming in here and infecting our people".
The entry restrictions will take effect from Thursday.
Despite the rising number of cases President Jair Bolsonaro, has regularly played down the seriousness of the disease.
He has repeatedly said that 70 percent of Brazil's population would eventually be infected with coronavirus and "there's no running away from that".
The actual number of cases and deaths is believed to be higher than the official figures disclosed by the government, as the testing capacity of Latin America's largest country still lags.