The SMRT bus drivers' action on Monday was an illegal strike and will be dealt with, says Acting Manpower Minister Tan Chuan-Jin.

Police are investigating.

He is speaking at an ongoing Ministry of Manpower press conference on the SMRT saga. Also present are Minister of State for Transport Josephine Teo and senior officials from the manpower and transport ministries.

A total of 102 China-born drivers refused to go to work on Monday, saying they wanted better salaries. They said they are paid less than Malaysian drivers.

On Tuesday, 60 continued to stay away from work. SMRT said earlier that some had "valid medical reasons".

Police deployed officers to SMRT's rented dormitory for drivers in Woodlands in the morning. A police spokesman said its men were there to observe the situation.