Marshall Islanders are at risk of serious medical complications if they are infected twice with different strains of dengue fever, the country's Health Secretary says.
The number of people with type 3 dengue on the island of Ebeye in the Marshalls has risen this week, with 158 people now infected.
In 2011, during the last outbreak of dengue in the Marshall Islands, there were 1600 cases of dengue type 4.
Currently, in the capital, Majuro, health authorities are awaiting test results to confirm the disease has spread beyond Ebeye.
Health Secretary Jack Neidenthal said two people in Majuro, who had arrived from Ebeye, had become unwell this week.
They arrived just before travel restrictions were imposed, he said.
"We have now discovered two suspect cases of dengue here on Majuro. We've sent them off for lab confirmation on Hawai'i. So we can't move forward with more drastic actions until that is confirmed. It only takes a single case of dengue fever where you have to declare an outbreak," Mr Neidenthal said.
Schools on Ebeye have been closed for two weeks and many of the unwell are children, he said.
Mosquito spraying on Ebeye was helping to slow the disease's spread, Mr Niedenthal said.
Further travel restrictions will be imposed if the Majuro cases are confirmed and an outbreak is declared in the capital, he said.
Results from the Majuro cases are expected in the next few days.