American Airlines' president on Friday said it is too early to tell if the rapidly spreading Zika virus will curb air travel, but so far the airline has seen no material change to its bookings.

American Airlines Group Inc., the world's largest airline, and other U.S. carriers are facing mounting concern about the mosquito-borne virus as investors mull a slump in demand to the Caribbean and other tourist hotspots.

Zika could put a further dent in revenues for American, which has the widest Latin American network among U.S. airlines. American President Scott Kirby said on a call with investors Friday that flight sales to Brazil had fallen to about 2 percent of its revenue from about 6 percent in the past two years.

While airlines have yet to report a bookings drop because of Zika, a warning from the World Health Organization that 4 million people in the Americas could get the virus has exacerbated jitters for the travel industry, hurt by outbreaks of SARS and Ebola in years past.

"People incrementally are starting to pay a little more attention to Zika than they were before," said Sterne Agee CRT analyst Adam Hackel. "(They) freak out when it comes to airlines."

New York-traded airline stocks dropped 3.1 percent Thursday, which analysts attributed to Zika fears and an uptick in oil prices, before rebounding Friday.

Within that sector, American saw the biggest stock decline Friday with shares falling more than 1 percent. It forecast a passenger unit revenue decline in the first quarter between 6 percent and 8 percent, not accounting for any potential Zika impact.

The virus has been linked to birth defects in thousands of Brazilian babies.

On Thursday, smaller rival JetBlue Airways Corp. was hit by investor selling "on fears around exposure to Zika-affected areas" because more of its revenue comes from the region than peers, Credit Suisse analyst Julie Yates said in a research note.

Like American, JetBlue said it has seen no measurable impact from the virus but would watch the issue closely.

Still, analysts noted the industry saw a sharp, if short-lived, sell-off in October 2014, when deadly Ebola was reported in the United States.

"Short-term especially, these stocks will react," Hackel said.

American is offering pregnant customers — who appear most vulnerable to the virus — and their companions refunds for tickets to Zika-impacted regions. United, Delta and JetBlue have also announced refunds.

American Airlines An American Airlines jet lands at Chicago O’Hare International Airport. Photo: Scott Olson/Getty Images