The head of the US Central Intelligence Agency said Tuesday that terrorism could not be ruled out in the disappearance of the Malaysian Airlines aircraft, describing the plane's fate as a "mystery."

CIA Director John Brennan said there had been reports of claims of responsibility for downing the missing jet, but stressed that these were far from confirmed.

"I think there's a lot of speculation right now -- some claims of responsibility that have not been, you know, confirmed or corroborated at all," he said.

"We are looking at it very carefully."

Brennan provided no further details, but his comment was the first reference by a US official to any alleged claim of responsibility over the jet's fate.

When asked if he could rule out a terrorist link, Brennan said: "No, I wouldn't rule it out."

The former counter-terrorism adviser to President Barack Obama stressed that it was too early to draw any conclusions about the Malaysia Airlines plane that vanished Saturday with 239 people on board after taking from Kuala Lumpur en route to Beijing.

"Clearly this is still a mystery," he said.

There were a host of unanswered questions including why the plane's transponder stopped emitting signals and what was the role of passengers carrying stolen passports, he said.

"There are a number of very curious anomalies about all of this...You know, did it turn around? You know, were the individuals with these stolen passports in any way involved?"

He added: "What about the transponder? Why did it sort of, you know, just disappear from the radar?"

He admitted that it was easy to speculate, but warned: "I think at this point we just have to, again, be patient and wait and let the authorities continue to investigate."

Asked about speculation that the pilot may have sought to commit suicide, Brennan said: "I think you cannot discount any theory."

Brennan's comments came at a rare public speaking appearance at an event in Washington organized by the Council on Foreign Relations, a think tank.

Malaysian authorities have doubled the search radius to 100 nautical miles (equivalent to 185 kilometers) around the point where the Boeing 777 disappeared from radar over the South China Sea.

Malaysia also has opened a terror probe, joined by FBI agents from the United States.

Officials said Tuesday that the two men who boarded the flight using stolen European passports appear to have been young Iranian migrants seeking a new life overseas.