LYON (AFP) - The mysterious disappearance of a Malaysia Airlines plane with 239 people on board was not likely caused by a terrorist attack, Interpol said Tuesday after revelations two passengers had used stolen European passports.

"The more information we get, the more we are inclined to conclude it was not a terrorist incident," Ronald K. Noble, head of Interpol, told reporters.

He pointed to declarations by Malaysian police that one of the two suspect passengers was a 19-year-old Iranian illegal immigrant.

"If you read what the head of police of Malaysia said recently about the 19-year-old... wanting to travel to Frankfurt, Germany in order to be with his mother, it is part of a human smuggling issue and not a part of a terrorist issue," Noble said.

The plane was travelling to Beijing, and the Iranian is believed to have been heading to Frankfurt via the Chinese capital. The final destination for the other ticket was Copenhagen, according to travel documents seen by AFP.

Authorities have doubled their search radius around the point where the plane disappeared from radar over the South China Sea early Saturday.