It was not escaped terrorist Mas Selamat Kastari, who broke out of a detention centre on Wednesday, but a Sri Lankan man in his late 20s.

The officers arrested the illegal immigrant at 6.30am when they were conducting stepped-up checks on vehicles leaving the country.

The Malaysian bus driver, who in his late 20s and said to be in cahoots with the stowaway, was also arrested.

The 40-seater bus was not carrying any passengers nor any luggage.

Deputy Superintendent Tan Kong Hui, the deputy commander for counter terrorism and security, said: 'When we first found him, it is natural that we had linked him to the terrorist because that is who we are all looking for.'

According to DSP Tan, preliminary investigations show that this illegal immigrant is not linked to Mas Selamat.

But he is the first suspicious character arrested trying to leave Singapore illegally at the Woodlands Checkpoint since the manhunt for the Jemaah Islamiyah militant started three days ago, said DSP Tan.

Immigration checkpoints in Singapore have been put on high alert for the missing terrorist.

At the Woodlands Checkpoint on Friday, police conducted thorough checks on all vehicles, including the boots. Drivers were also requested to step out of their vehicles.

Before the high alert, only random vehicles were searched. Now, every vehicle is put through a check. DSP Tan said: 'We will examine each and every vehicle and not spare a single one.'

Such stepped-up checks have caused some delays for travellers who wait to clear the checkpoints.

There were massive traffic jams at both checkpoints in Tuas and Woodlands during the evening peak hours on Thursday, with some waiting for as long as an hour to get to Malaysia.

There was also a slight jam at the Woodlands Checkpoint on Friday morning.

DSP Tan appealed for the public to be 'more understanding' and emphasised the importance of their tight security checks.

'Every one should be aware of the current urgent need to enforce such stringent checks so we seek for the public's full cooperation.'