The Ministry of Manpower has told bus operator SMRT that labour and contractual grievances raised by its workers must be a priority and addressed quickly.

SMRT must ensure lines of communication are kept open and that it has proper grievances handling procedures in place.

The ministry revealed this in a statement in response to queries about the SMRT bus drivers' illegal strike.

Four drivers from China were charged in court earlier on Thursday for their role in the illegal strike at a worker's dormitory in Woodlands earlier this week.

The ministry also said it has inspected the dormitories where the mainland Chinese drivers were housed. They had complained of poor conditions.

The ministry said the rooms were not overcrowded, but there was a bedbug problem in some of those occupied by the drivers. "Occupants of each room are responsible for their own hygiene," it said.

The "general housekeeping conditions" of the drivers' rooms were "below par" compared to other rooms in the dormitories.

SMRT management said in a statement it has held two dialogue sessions with more than 130 bus drivers from China. They were told about what measures were being implemented to improve their well-being. including fumigation of bedbugs and the repair of fixtures.