US health officials' concerns continue to mount as two more pregnant women in the country have been found to be carrying the mosquito-borne Zika virus linked to serious birth defects.

Two pregnant women from Illinois have the virus, which officials believe they caught outside of the US. Three other travellers living in Florida, who are not pregnant, have also been found to have the virus, also contracted from another country.

It follows the case of a woman in Hawaii who recently gave birth to a baby girl with microcephaly, the condition of having an unusually small head and brain, which scientists believe can happen when a pregnant woman has contracted Zika.

The mother apparently contracted while in Brazil during the early part of her pregnancy.

American officials fear the mosquito-borne virus could flourish in the moist, warm climates of places like Hawaii and the coastal south. Zika is believed to have caused thousands of cases of microcephaly in Brazil.

Officials of the US Centers for Disease Control are warning pregnant women not to travel to many nations in Central and South America, with the virus. Now, officials are advising American doctors to ask all their pregnant patients if they have recently travelled to a country with a Zika outbreak.

Zika virus: The mosquito-borne disease explained in 90 seconds IBTimes UK

The CDC is also advising any pregnant women who have travelled to countries with Zika to get tested for the virus if they are have two or more symptoms of the disease.

Often mild symptoms are ignored, but Zika can have a devastating impact on the foetus of a pregnant woman and the virus has no cure or vaccine.

The new guidelines will help protect pregnant women and their babies-to-be, Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center, told ABC.

But it will also "provoke a fair amount of anxiety in the pregnant women and their partners," he said. "I empathise with them, as well as my colleagues in obstetrics." In the event of contagion, doctors "don't have anything to offer except compassionate care," he noted.