Eighteen Americans evacuated from a cruise ship after possible exposure to hantavirus are being closely monitored by health officials, as authorities maintain the risk to the public remains "very, very low".

Officials say one passenger aboard the Dutch vessel MV Hondius has tested positive for the Andes virus - a rare type of hantavirus - while another is showing mild symptoms.

More than 90 passengers of the cruise ship, currently docked in Spain's Canary Islands, are being repatriated.

The positive result marks the first confirmed case involving an American passenger. Two people are being monitored in Atlanta, while 16 others are in Nebraska at the nation's only national quarantine unit.

"No-one who poses a risk to public health is walking out the front door onto the streets of Omaha," Nebraska Governor Jim Pillen said at a press conference alongside health officials on Monday morning.

In an effort to preserve space at the facility in Nebraska, some passengers were flown to Atlanta, including the passenger who displayed mild symptoms, said Brendan Jackson of the Centers for Disease Control and Prevention.

The two individuals taken to Georgia include one who is symptomatic and that person's partner.

"Let me be crystal clear: the risk of hantavirus to the general public remains very, very low," said Admiral Brian Christine of the US Health and Human Services department (HHS). "The Andes variant of this virus does not spread easily, and it requires prolonged close contact with someone who is already symptomatic."

Experts say most strains of hantavirus, a group of viruses carried by rodents, do not pass from person to person, but the Andes strain identified in a number of people who had been on the Dutch cruise ship does.

The 16 passengers who were taken to Nebraska are in "good shape" and "good spirits", Michael Wadman, medical director of the National Quarantine Unit, said.

He said the person who had tested positive to Andes virus was placed in a biocontainment facility and was not experiencing any symptoms.

The CDC cautioned that people could have symptoms, but that did not mean that they necessarily had the hantavirus. Any symptoms of a mild cold would count as a symptom, Brendan Jackson of CDC said, noting that they were being extra cautious.

When health officials announced that one individual tested "mildly positive" on a PCR test, it raised questions about that terminology.

That patient's specimen was taken on the ship, not in the US, Jackson said noting that there were two specimens collected from that patient, one of which was positive and the other negative.

"With these PCR tests... there's sort of a range in where they can fall," he said at the press conference. "And so for that reason, we just want to make sure there's further testing to evaluate that."

Health officials are now focused on "symptom monitoring". Wadman noted that the passengers in Nebraska would undergo further assessment when they have had a chance to sleep and rest.

Passengers are expected to remain at the Nebraska facility for assessment over the next several days. Officials will then determine on a case-by-case basis whether they need to complete the full 42-day quarantine period.

Three people have died following the outbreak on the MV Hondius, including two whom the World Health Organization has confirmed had hantavirus.

Two British nationals, who are being treated in the Netherlands and South Africa, also have confirmed cases.

A British-US dual national is currently in quarantine in Nebraska.