A South Korean city plans to introduce an alarm that flashes a pink light on trains and buses to prompt commuters to give up their seats to pregnant women.

The Beacon device uses bluetooth technology and lights up beside priority seats when a pregnant woman, with the associated app on her smartphone, moves within 2 meters (6.6 feet) of the alarm.

The device drew praise from commuters after a trial was launched in April on trains between the country's second biggest city of Busan and the neighboring city of Gimhae, as it helped identify whether a woman was pregnant.

"It is hard to tell if they are pregnant when they are in the early stages," said 24-year-old passenger, Han dong-jin.

"In this respect, it is a very nice campaign."

City officials have said they hope to widen the program to more train and bus lines by the end of the year.

(Reporting by Dagyum Ji and Sanggyu Lim; Editing by Patrick Johnston)