People wear masks after deaths and new confirmed cases revealed from the coronavirus in Tehran, Iran on February 21, 2020.

The new coronavirus that is rapidly spreading around the world entered Iran "unseen and undetected," world health officials said.

Iran, which had just two cases a week ago, confirmed 245 infections as of Thursday morning, but the outbreak could be even more widespread in the country than is currently known, said Dr. Mike Ryan, executive director of the World Health Organization's emergencies program.

"This disease came unseen and undetected into Iran, so the extent of infection may be broader than what we may be seeing," Ryan told reporters at a news conference at the agency's headquarters in Geneva on Thursday.

Kianoush Jahanpour, a spokesperson for Iran's health ministry, on Thursday raised the country's death toll due to the virus to 26. That means Iran has recorded the highest number of coronavirus fatalities outside China.

As epidemics emerge, he added, the health-care system often only picks up the most severe cases because those are the individuals who are likely to seek treatment. He said the broader spread of cases is likely due to surveillance difficulties, rather than the efficacy of Iran's treatment and isolation protocols.

At the start of an epidemic, "you only see the severe end of the spectrum and then as you do more surveillance you find more mild cases," he said. "I suspect this has more to do with surveillance and detection of cases at this point."