Unionized Seoul bus drivers said Wednesday that they were planning to strike over pay starting Thursday, if no agreement was reached at last-minute negotiations with their employers.

The Seoul Bus Labor Union, which has nearly 18,133 members across 61 companies, voted Tuesday on a plan to strike at 4 a.m. Thursday. According to the union, 16,317 members participated in the vote, and 16,046 of them voted in favor of the strike.

The union, which is part of the Korean Automobile and Transport Workers' Federation, decided to hold last-minute negotiations with employers on Wednesday afternoon and to go on strike if an agreement was not reached by midnight.

The union has demanded wage hikes, a revision of the salary system and the abolition of compensation discrimination against contract workers since late December. The Seoul city government, which oversees the management of bus companies, has so far accepted none of their demands, the union said.

Over the past five years, the average wage of bus drivers in Incheon has risen 5.54 percent annually, but the average wage of bus drivers in Seoul has risen only 2.98 percent a year, the union said.

The union stressed that, despite seven rounds of negotiations over the past three months, the differences have not narrowed between the union and the city government.

If they decide to walk out, 98 percent of buses in Seoul will be affected, the union said. It will be the first bus strike in the city in 12 years.

The Seoul city government said Wednesday it would carefully monitor how negotiations proceed, adding that it would prepare emergency measures in case the union goes on strike.

These emergency measures include increasing the number of subway trains, especially during rush hour, extending subway operations to 2 a.m., and deploying maintenance personnel to 17 highly congested stations, such as Jamsil Station, Sadang Station and Seoul Station.

Also, the city's 25 districts plan to operate free shuttle buses. A total of 400 vehicles are to be deployed. Additionally, traffic information will be provided through the 120 Dasan Call Center, the Topis Seoul traffic information center website, the Seoul City website and the city's official social media account.