SAO PAULO (REUTERS) - The entire Brazilian state of Espirito Santo is now considered at risk for yellow fever transmission, the World Health Organization says, raising concerns the deadly virus could spread to the nation's biggest cities.

An ongoing yellow fever outbreak has so far been limited to rural areas, where it is mainly spread to humans by two rural species of mosquito that likely bit infected monkeys, according to Brazilian health officials.

But there are growing concerns the virus could spread to urban centres like Rio de Janeiro, Sao Paulo, Belo Horizonte and Vitoria - areas where tens of millions live and where the WHO said the virus could likely start spreading in a human-to-human cycle via the Aedes aegypti mosquito.

That mosquito, common in many tropical and subtropical cities, also carries the dengue and Zika viruses.

In a 2013-17 risk assessment, the WHO advised travellers to get vaccinations if they were visiting all but eight Brazilian states - six in the northeast, along with Espirito Santo and Rio de Janeiro. The WHO list now includes northern Rio de Janeiro and all of Espirito Santo, according to WHO guidance updated this week.

Brazil's Health Ministry said Tuesday (March 7) that since December, when the yellow fever outbreak was first detected, there have been 127 confirmed deaths, with another 106 under investigation.

Out of a total 1,500 suspected cases, 371 have been confirmed, 966 are still being examined, and the rest ruled out. The number of suspected and confirmed cases is Brazil's highest since 2000.

Vaccination against yellow fever fell in Brazil's southeast in recent decades because the virus had in large part been eradicated from the region. Now, though, the health ministry is rushing to vaccinate people, sending nearly 15 million extra doses to the newly affected areas.

A viral disease found in tropical Africa and the Americas, yellow fever mainly affects humans and monkeys. It is often asymptomatic or mild in humans, but is deadly for monkeys.

A sharp increase in the number of dead monkeys found in Brazil's southeast first raised alarm the virus is spreading, especially after a few confirmed cases in monkeys occurred near Belo Horizonte and Vitoria.

It is not yet clear what sparked the outbreak.

Brazil's health officials are investigating whether it is related to a tailings dam collapse last year at an iron ore mine owned by BHP Billiton and Vale SA.

The accident, Brazil's worst ever environmental disaster, could have disrupted monkeys' habitat and food supplies.