The World Health Organization's top emergency expert, asked on Thursday about contradictory remarks by President Donald Trump and U.S. health officials on COVID-19, said it was important for all countries to have "consistent messaging" for their public.

Trump took exception on Wednesday to comments from the director of the U.S. Centers for Disease Control and Prevention, Robert Redfield, who said a vaccine could be broadly rolled out in mid-2021 and masks might be more effective.

Trump, at a news conference, said he believed a vaccine will be rolled out much sooner. He said he called Redfield after his testimony to question him about it, and that Redfield appeared to have been confused by the question.

"It is important that we have consistent messaging from all levels, and it's not for one country or one entity; consistent messaging between science and between government," the WHO's Mike Ryan said in response to a question about the exchange.

The science around the coronavirus was "complicated stuff", with data and new evidence evolving, Ryan said. "So it isn't easy and it isn't easy for everyone to be on message all of the time."

"What is important is that governments, scientific institutions, step back, review the evidence, and give the most comprehensive easy-to-understand-and-digest information so that people can take the appropriate action."

Ryan, a veteran of Ebola outbreaks in Africa and other epidemics, said authorities must engage with communities to address anxieties.

"It's understanding the confusion, it's understanding their concern, it's understanding their apprehension. And not laughing at it and not turning that into some kind of political football," he added.

Maria Van Kerkhove, the WHO's technical lead for COVID-19, said it recommended a comprehensive package of measures including physical distancing to prevent spread of the virus.

"Masks are part of it," she added.