SINGAPORE - The immigration clearance system at Woodlands and Tuas checkpoints is experiencing "intermittent slowness", said the Immigration and Checkpoints Authority (ICA) in a statement.

The system has been experiencing slowness since 5.40pm on Tuesday (May 16). The ICA, which did not specify the cause of the problem, said investigations are ongoing.

"As this coincides with the departure peak period, more resources have been deployed to manage the situation at the land checkpoints," said the statement. "The ICA regrets the inconvenience caused to travellers and seeks their understanding and patience."

When contacted, an ICA spokesman told The Straits Times that the problem was not due to a cyber attack, citing preliminary investigations.

The slowness resulted in slower processing time and congestion, the spokesman added.

All available resources were deployed to manage the situation, with both checkpoints running at full capacity. Officers in the afternoon shift were working overtime, while night shift officers were asked to arrive early.

These steps have helped improve the time needed to clear the checkpoint as of 11.10pm, said the spokesman.

"The immigration clearance system is progressively resuming normalcy," said the ICA in an update to an earlier Facebook post announcing the delay.

"Arrival and departure traffic at both land checkpoints is improving but still heavy."

Travellers were advised to postpone non-essential travel.

Mr Al Furqan, a 29-year-old property executive, told The Straits Times on Tuesday (May 16) evening that he was at the checkpoint at 7.30pm on his motorcycle.

"I reached Johor Baru customs only at 10pm," he said.

Mr Furqan, who shared a photo of motorcycles stuck in a jam at Woodlands Checkpoint at 7.40pm, said jams usually take about 1.5 hours to clear but it took twice longer on Tuesday.