Government officials won't be the only ones enforcing the Trump administration's latest curbs on travel to the U.S. when they take effect next month.

Commercial airlines are again on the front lines of the U.S. government's latest round of travel restrictions, which have had several iterations since the Trump White House's first, issued in January. The first set, placed upon citizens of seven majority-Muslim nations, threw airports into chaos.

Failing to scrutinize traveler documents is costly: Airlines have to pay to fly the passengers back home and sometimes pay hefty fines for passengers who are denied entry into the U.S. upon landing.

Immigration authorities worldwide turn away some 60,000 air travelers a year at destination or transfer points, according to the International Air Transport Association, an industry group. Those authorities can fine carriers an average of $3,500 for the error.

Gate agents and other airline employees this year have had to become quick studies in the sometimes vague and abruptly changing requirements on which documentation was acceptable for boarding U.S.-bound flights. They also were forced to change their boarding and luggage-loading procedures after the Trump administration in March issued a ban on personal electronics larger than a cell phone in the cabin.

The White House's latest rules added a ban on most U.S. travel from Chad and North Korea, to a list that included Syria, Yemen, Somalia, Libya, and Iran. Sudan was removed from the list of countries that were in an earlier ban.

The administration also will bar the entry of some government officials and their families from Venezuela. The new restrictions will take effect Oct. 18, the White House said.

Unlike under previous travel bans, the restrictions differ country by country, which means airline employees need to review passenger documentation ever more carefully. For example, Iranian students with valid visas to study in the U.S. will still be admitted.

There are no scheduled nonstop flights from this list of countries, except Venezuela, but there is connecting service to the U.S. through airports in the Middle East and in Europe, so not only regional carriers will be affected.