Top WHO official warns cruise passengers are 'high-risk contacts' as Americans return from hantavirus ship

"Our recommendations are we're considering everyone on board, the crew and the passengers as high-risk contacts because there are current investigations that are really trying to understand what types of exposures each of them had," said Maria Van Kerkhove, acting director of the Department of Epidemic and Pandemic Threat Management at WHO.

"Because there's so much uncertainty at the moment, we've recommended that everybody be considered a high-risk contact."

According to Reuters, Spanish officials said passengers are being tested by local health authorities to ensure they are asymptomatic before being transported ashore in small boats.

Spanish health authorities confirmed that the first plane carrying the Spanish passengers has departed for a military hospital in Madrid, where they will remain under quarantine.

Kerkhove said WHO's recommendation is that a 42-day active follow-up period succeed passengers' return to their respective home countries.

"This would include health checks by authorities and either be quarantined at home or quarantined in a medical facility," she explained.

They will instead be screened upon arrival in the U.S. and either stay briefly at Nebraska's National Quarantine Unit or return home to monitor for symptoms for 42 days while staying in contact with local health authorities, the official said.

Fox News' Michael Sinkewicz contributed to this report.