More than 100 mainland Chinese bus drivers in Singapore refused to work on Monday in a rare case of labour mass action in the city-state. The drivers, disgruntled over their pay from state-linked transport firm SMRT, refused to board a shuttle that was going to ferry them from their dormitory to a nearby depot. After talks with SMRT management with police on standby, the protesting drivers said they would report for work on Tuesday. SMRT is 54 percent owned by state investment firm Temasek Holdings. Singapore has been hiring bus drivers from China and Malaysia because of a chronic shortage of manpower. One of the Chinese drivers, who declined to be named, told reporters the dispute arose because they felt aggrieved over a disparity in pay between Chinese and Malaysian bus drivers. Drivers from China earn a basic salary of Sg$1,075 ($879), while those from Malaysia earn $1,375, the driver said. The dispute was also about the lack of bonuses for Chinese bus drivers, he added. Strikes and other forms of industrial action are rare in Singapore, where the labour movement works closely with the government and private business, making the city-state an attractive place for foreign investment. The Ministry of Manpower issued a stern warning to the 102 drivers who took part in the stoppage, saying it "takes the workers' actions very seriously" and was closely monitoring the situation. In a statement, the National Transport Workers' Union urged the Chinese drivers -- who are not union members -- to return to work immediately. No major disruptions were reported during the work stoppage, which took place during Singapore's year-end school holidays, when demand for public transport is lower.