NEW DELHI: Heightened security arrangements prompted by increased threat perceptions following the Mumbai serial blasts caused chaos at the Delhi airport and led to domestic flight delays of up to four hours on Thursday, causing immense hardship to passengers.

Serpentine queues were witnessed at the Delhi Airport security check-in with the result that passengers were held up and airline schedules delayed.

At least ten morning flights of private carriers were delayed considerably, including a Jet Airways flight to Mumbai by four hours.

Civil Aviation Secretary Ajay Prasad said the delays were caused by additional security measures that were put in. The situation was compounded by the malfunctioning of an X-Ray machine at Terminal 1-B, which caters to private airlines, including Jet Airways, Air Sahara, SpiceJet and Air Deccan.

"We held a detailed review meeting with airport officials and those from the GMR Group. We have decided to simplify the security procedures without compromising security," he said after an hour-long meeting which was also attended by AAI chief K Ramalingam, Bureau of Civil Aviation Security chief S R Mehra and representatives of DGCA.

"There are serious threat perceptions about airports. ... We have some information that airports could also be (terrorist) targets. It is therefore necessary to take all preventive measures," Prasad said, without giving details.

He also advised passengers to avoid last minute check-in and arrive early at the airport.