A Civil Aviation Ministry spokesperson said all airports have been on alert for the past four days due to the threat perception.

The sources said the Bureau of Civil Aviation Security has instructed the CISF to upgrade security measures at the airports, including introducing additional checks and searches and enhancing the strength of personnel.

The BCAS decision came in the wake of intelligence inputs that some Kashmiri militant outfits were likely to target passenger aircraft and try to hijack one.

The spokesperson said the BCAS was periodically reviewing the threat perceptions and would maintain the high level of security till such perceptions died down.

All Indian airports have been on alert since the serial blasts on Mumbai suburban trains on July 11, 2006.