SINGAPORE - Prime Minister Lee Hsien Loong said he is "very concerned" about the major disruption to train services on Tuesday because the root cause of the problem has not been identified and it may recur.

In a Facebook post on Wednesday, Mr Lee said he has been briefed about the disruption to train services, which lasted more than two hours, on the North-South and East-West lines during Tuesday's evening rush hour.

"I had just recently visited the SMRT Bishan Depot, to see their maintenance operations and teams. So I went to the LTA Ops Centre this afternoon, to be briefed on the situation," he wrote.

"We are still trying to find out the cause of the problem. LTA and SMRT staff and engineers worked through the night checking trains, tracks and cables," he added.

The Land Transport Authority (LTA) and operator SMRT said a faulty train could have caused the power to trip. At a press conference on Wednesday, SMRT said the first traction of power tripping occurred at 5.59pm. Within a few minutes, 20 counts of traction power trip occurred at multiple locations across the network.

SMRT managed to reset the trippings and commuter services were maintained. At close to 7pm, SMRT realised that the trip could not reset and a total of five trains had stalled between stations. Within a few minutes, two more trips happened.

SMRT then decided to suspend train service for the sake of passenger safety.

Full train service resumed on Wednesday without incident.

PM Lee, however, noted: "But because we have not identified the root cause of the power trips, we are still very worried that the problem may recur. Hope we identify and resolve the faults quickly, to prevent further inconvenience to commuters."

SMRT and LTA have since identified three possible causes for the power trips. Engineers found that the insulation on two power cables along the North-South line were damaged. There was also a faulty voltage relay at the Kranji power substation and water leakage along a section of the Tanjong Pagar tunnel . All these damaged components have been fixed or replaced.