Two crane operators staged a high-rise protest at a construction site in Singapore on Thursday, its second industrial incident involving mainland Chinese workers in less than two weeks.

The workers, reportedly demanding payment of back wages before their return to China, perched themselves on top of separate cranes before agreeing to come down after hours of negotiations with rescuers, an AFP photographer said.

The crane protest came 10 days after a work stoppage by Chinese bus drivers erupted at a transport firm -- Singapore's first industrial strike since 1986.

The protest came as four drivers accused of instigating the November 26-27 strike at state-linked transport firm SMRT appeared in court.

Liu Xiangying, 33, Gao Yue Qiang, 32, Wang Xianjie, 39, and He Jun Ling, 32 appeared in handcuffs and told the court they would engage defence counsel.

Twenty-nine other drivers have already been fired and sent back to China and one has been sentenced to six weeks in jail after pleading guilty to involvement in an illegal strike.

Activist groups have expressed outrage over the crackdown on the bus workers.

Human Rights Watch (HRW) issued a statement Thursday calling on Singapore to drop the charges against the four drivers and put an end to pay discrimination.

The striking drivers complained they were being paid less than Malaysian colleagues for the same work.

"Singapore defies basic labour rights by criminalising migrant workers for a work stoppage and threatening them with prison terms, fines, and deportation," said Phil Robertson, HRW deputy Asia director.

"As a country that depends on migrant workers, Singapore should recognise it's playing with fire by permitting private and state-owned companies to discriminate based on the nationality of the workers," he added.

Involvement in an illegal strike in Singapore is punishable by a maximum one-year jail term and Sg$2,000 ($1,640) fine, or both.

Last week's bus strike highlighted the country's heavy dependence on migrant labour to drive its economic growth amid a labour shortage resulting from falling birth rates.