In its most detailed update on the outbreak, published late on Monday, the UN health body said the risk to the wider public was low from a disease typically spread from infected rodents that only rarely passes between humans.
But it said limited human-to-human transmission had been reported in some strains of the virus.
The authorities in Cape Verde - an island nation in the Atlantic off West Africa - said they had not allowed the Dutch-flagged MV Hondius to dock as a precaution.
About 150 people are stuck on the Hondius, which was carrying mostly British, American and Spanish passengers on a luxury cruise that set off from the southern tip of Argentina in late March.
The cruise visited the Antarctic peninsula and South Georgia and Tristan da Cunha - some of the remotest islands on the planet.
The seven confirmed or suspected cases comprised three people who had died, one who was critically ill and three with mild symptoms, the WHO said.
The three fatalities were a Dutch couple and a German national, while a British national was evacuated from the ship and is in intensive care in South Africa, officials said.
As a precaution, passengers aboard the Hondius have been instructed to remain inside their cabins whenever possible, the WHO said, adding that the incubation period could last several weeks, meaning some people might not yet be showing symptoms.
Epidemiological investigations were under way to determine the source of the outbreak, the WHO said.
Medical teams in Cape Verde are evaluating the patients and collecting additional specimens for testing.
The WHO said that although uncommon, limited human-to-human transmission of the Andes virus, a species of hantavirus found in Argentina and Chile, "has been reported in community settings involving close and prolonged contact".
The Dutch couple who were the first to die had travelled in South America, including Argentina, before they boarded the cruise ship.
The ship's operator, Oceanwide Expeditions, said authorities were working on a plan for a medical evacuation of two people on board who are ill, as well as person who travelled with one of the passengers who has died.
There wais no definitive plan yet for the disembarkation of the remaining guests on board, the company said.
It only confirmed that this would not be done in Cape Verde.
A spokesperson for Spain's health ministry told Reuters they had not yet received a request to dock at the Canary Islands.
The Hondius left Ushuaia in southern Argentina in March, according to company documentation, on a voyage marketed as an Antarctic nature expedition.
The first stricken passenger, the Dutch man, died on April 11.
His body remained on board until April 24, when it "was disembarked on St Helena, with his wife accompanying the repatriation", Oceanwide Expeditions said.
His wife, who had gastrointestinal symptoms when she was disembarked, later deteriorated during a flight to Johannesburg.
She died upon arrival at the emergency department on April 26, the WHO said, adding that contact tracing was under way for passengers on that flight.
South African authorities have confirmed that the British patient, who is being treated in a Johannesburg hospital, tested positive for the hantavirus.
The Netherlands has confirmed the virus in the Dutch woman who died.
