By Julie Steenhuysen

CHICAGO (Reuters) - Health authorities in Florida have tested about 200 people for the Zika virus as part of the state's investigation of two possible cases of infections not related to travel to a region hit by an outbreak, officials said on Friday.

Florida's health department this week began looking into what may be the first cases in the United States of Zika, a mosquito-borne virus that has spread rapidly through Latin America and the Caribbean, caused by the bite of a local mosquito.

All prior cases of Zika in the United States have been linked with travel to a country where the virus is circulating or to sex with someone who has done such travel.

Health experts in the United States have been watching closely for the arrival of Zika, which has been shown to cause a birth defect called microcephaly marked by small heads and undersized brains that can cause severe developmental problems.

Florida health officials have urged people in the areas being investigated to provide blood and urine samples if requested.

"The department's investigations into the possible non-travel related Zika virus cases in Miami-Dade and Broward counties are ongoing and the department will share more details as they become available," the department said in a statement. "To date, approximately 200 people have been interviewed and tested as part of the department's investigations and we await additional lab results."

On Tuesday, Florida said it was investigating a possible Zika case in a Miami-Dade County woman. The case is suspected to be non-travel related, but as of Thursday the state had not ruled out travel or sex as a potential cause of her infection, according to a spokeswoman.

Late on Thursday, Florida also announced a potential Zika case in Broward County that was not related to travel.

Florida has invited the U.S. Centers for Disease Control and Prevention to assist in the probe, and CDC senior medical epidemiologist Dr. Marc Fischer arrived on Friday.

To determine whether a case was caused by local transmission, health workers need to survey households and neighbors within a 150-yard (137 meters) radius of an infected person's residence, Fischer said at a CDC workshop on Thursday.

He said that was basically the flying radius of the mosquitoes that spread Zika.

More than 400 pregnant women in the continental United States have evidence of Zika virus infection, and 12 infants have been born with birth defects linked to Zika infections in their mothers, officials have said.

(Reporting by Julie Steenhuysen; Editing by Will Dunham)