US President Barack Obama has called for faster research on the quick-moving Zika virus, which is spread by mosquitoes and has been linked to a rise in birth defects in Brazil.

Obama on Tuesday urged better diagnostic tests and the development of vaccines and treatments against the virus, which the World Health Organization has said is likely to spread throughout the Americas.

As of now, there is no vaccine or medicine to treat Zika virus, and no way to prevent it other than by trying to avoid mosquito bites.

Obama was briefed on the situation by top science experts in the US government, including the US Centers for Disease Control and Prevention, Health and Human Services, and the National Institutes of Health, according to a White House statement.

On Tuesday, the CDC expanded its travel warning for pregnant women and those considering becoming pregnant to avoid 24 areas in Latin America and the Caribbean that have seen cases of Zika virus.

Now, travelers are advised to postpone visits to the US Virgin Islands and the Dominican Republic, along with Puerto Rico, Barbados, Bolivia, Brazil, Cape Verde, Colombia, Ecuador, El Salvador, French Guiana, Guadeloupe, Guatemala, Guyana, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Saint Martin, Samoa, Suriname and Venezuela.

There have not yet been any cases of local transmission of Zika virus within the United States, although infected travelers have returned to the country after visiting other areas.

Zika virus is spread to people primarily through the bite of an infected Aedes species mosquito, according to the CDC.

Symptoms are usually mild and may include fever, rash, joint pain and conjunctivitis.

However, the virus can pass from a pregnant women to her fetus, and global health authorities are concerned by an apparent link between Zika virus and nearly 4,000 cases of babies born with unusually small heads -- a condition known as microcephaly -- in Brazil.