Security has been strengthened across Kerala. (Representational)

A man in Kerala detained under suspicion that he helped terrorists enter Tamil Nadu does not appear to have links to the security threat, the police said on Saturday. A companion of the suspect was also questioned in the incident, they said.

The police in Kerala and Tamil Nadu are on high alert following intelligence reports that six terrorists from Pakistan-based group Lashkar-e-Taiba have entered Tamil Nadu. One of them is believed to be Pakistani and others from Sri Lanka.

"We at this point do not want to divulge any kind of information that will identify the suspects since it can hamper the larger investigation," an intelligence official told NDTV, requesting not to be named.

Security has been strengthened around airports, railway stations, bus stands, places of worship and other public places in Kerala.

Kerala Police have urged the public to report anything suspicious on 0471 2722500.

In Tamil Nadu, security has been strengthened in Chennai and Coimbatore, with hotels, airports, railway stations, theatres, malls and places of worship under close watch.

At least 2,000 police personnel have been posted around Coimbatore, where a red alert has been sounded. According to news agency IANS, the LeT terrorists are believed to be hiding in the city.

Coimbatore has come attack from fundamentalist groups in the past. In 1998, serial blasts coinciding with the visit of senior BJP leader LK Advani killed 58 people and left 200 injured.