Iran offered its assistance Tuesday with a Malaysian investigation into two of its nationals believed to have been travelling on stolen passports on an airliner missing for days.

Foreign ministry spokeswoman Marzieh Afkham said Iran was "following up on reports regarding the possibility of two Iranian passengers aboard the plane."

"We are offering our cooperation to obtain more information," she said, pledging that Tehran would provide "any information on the Iranians and their status as soon as it is available."

Malaysian police said Tuesday that one of two suspect passengers aboard the missing jet was an Iranian illegal immigrant, as hopes for the 239 passengers and crew ebbed away.

Fears of terrorism were stoked by the weekend revelation that two men boarded the flight using stolen European passports. But police said people-smuggling was emerging as the likeliest explanation for the identity fraud.

One of the pair has been identified as 19-year-old Iranian Pouria Nour Mohammad Mehrdad, Malaysia's national police chief Khalid Abu Bakar told reporters.

"We believe he is not likely to be a member of any terror group and we believe he was trying to migrate to Germany," Abu Bakar said, adding that authorities had not yet identified the other man.

Authorities have doubled the search radius to 100 nautical miles around the point where Malaysia Airlines MH370 disappeared from radar over the South China Sea early Saturday, en route from Kuala Lumpur to Beijing.

With the aviation industry and authorities baffled over the plane's fate, investigators say they are ruling nothing out over how a huge Boeing 777-200 jet could have completely vanished.