The U.S. faces a "very difficult situation" in grappling with an epidemic that varies by state and is evolving at different rates across the vast country, a top World Health Organization official said Monday.

"I think that the United States has been dealing for a while with a complex situation," added Dr. Mike Ryan, executive director of WHO's emergencies program. "It's a very large country, 50 states, each one with different populations, with different levels of urbanization and the epidemic at different levels of development and evolution in each of those."

Ryan commended the U.S. for rolling out data-driven federal guidelines such as President Donald Trump's "Opening Up America Again" plan, which lays out three "phases" to guide states on how and when to reopen, based on factors such as new daily infections and hospital capacity.

"The federal government and the system of governors are working together to move America and its people through this very difficult situation with public health and other scientific leaders adding and inputting their advice into the system," Ryan said. "We believe that the overarching federal plan seems to be very much based on science."

Last week, Trump asked at a nationally televised White House press briefing whether injecting disinfectants into the body could be a Covid-19 treatment worthy of research. His comments drew the ire of toxicologists and public health specialists who called the comments reckless.

And earlier this month, Trump announced the U.S. will suspend funding to the WHO while it reviews the agency's response to the Covid-19 pandemic, adding that the international health agency made mistakes that "caused so much death."

Ryan's remarks come as states across the U.S. announce plans to reopen parts of the economy. Last week, Vice President Mike Pence said 16 states had unveiled "formal reopening plans."

Georgia Gov. Brian Kemp began to reopen parts of the economy on Friday, starting with retail locations such as gyms, barber shops, fitness centers and bowling alleys despite Trump saying he "totally disagrees" with restarting those businesses first.

Some public health specialists have expressed concern that parts of the U.S. are lifting restrictions too quickly and fear that the virus could reassert itself. The U.S. is still reporting roughly 30,000 new infections and nearly 2,000 deaths due to Covid-19 every day, former Food and Drug Administration Commissioner and CNBC contributor Scott Gottlieb told CNBC on Monday.

"The reality is there is still pervasive spread of coronavirus across the entire nation," Gottlieb, who sits on the boards of Pfizer and biotech company Illumina, said on CNBC's "Squawk Box," adding that some parts of the country never saw a major outbreak. "But for most parts of the country, they've plateaued but they haven't really started to show sustained declines in new cases."