U.S. tourism has slowed considerably since the inauguration of President Donald Trump, according to a study published by the searching app Foursquare, as airline industry lobbyists grow increasingly concerned over security measures on trans-Atlantic flights.

Foursquare said the data, mined from 50 million monthly users of its app across 150 countries, indicated that U.S.-bound tourism began to slow in October, from a global market share of around 6% to -16% by the end of March. Foursquare said California had been hit the hardest by the decrease in international travel, and indicated the U.S. -bound business trip activity, while improving, hasn't kept pace with the worldwide average.

"Observers of the above charts will note that the downturn in tourism came months before the new President came into office, and before changes to visa procedures, restrictions on travel from certain Muslim countries, the ban on certain electronics during flights from select countries and more," Foursquare said in a blogpost that accompanied the data.

"This timing may align with the heated rhetoric of the height of the Presidential election last fall, as the big dip began in October. International travelers may have determined that the America within their sights was less appealing or welcoming. Trend lines become even more steep in January when Trump took office, and have continued up through the end of Q1 2017," the blog posted noted.

The data follows recent figures from the International Air Transport Association (IATA), a lobby group which represents around 265 airlines comprising 83% of global air traffic, which showed improving revenues and traffic figures but cautioned that a ban on laptop computers on international flights could yet impact the industry's fragile recovery.

"The first quarter results are strong. But the last weeks have been challenging to the passenger business," said IATA Director General Alexandre de Juniac. "The laptop ban-implemented with next to no notice, no dialogue and no coordination, is testing public confidence in how governments and industry work together to keep flying secure."

"So, even as rumors persist that the ban will be expanded to other airports and regions, we are calling on governments to work with the industry to find alternatives-to keep flying secure without such great inconvenience to our passengers," de Juniac said.

A study published last month by Tourism Economics, a think-tank, pegged the cost of President Trump's plans to ban travel from certain, mostly Muslim, countries and the increased security measures could cost the U.S. economy more than $18 billion and put more than 100,000 jobs at risk.

The concerns also come at a difficult time for U.S. airline industry, which is struggling with rising fuel and labor costs alongside a series of embarrassing public incidents over the mis-treatment of passengers.

American Airlines Group Inc. (AAL) - Get Report reported first quarter earnings per share of $0.46 last month, down nearly 60% from the same period last year, even as revenues increased to $9.6 billion. United Continental Holdings (UAL) - Get Report , the parent of United Airlines, saw a similar trend, with rising costs eating into a year-on-year increase in revenues, resulting in a 65% year-on-year decline in first quarter earnings per share.

The S&P Composite 1500 Airlines index, a sector benchmark, has risen 4.4% so far this year compared to a 7.4% advance for the broader S&P 500.