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Iran has begun building a copy of the US surveillance drone it captured last year, after breaking its encryption codes, Iranian officials say.

"The Americans should be aware to what extent we have infiltrated the plane," General Amir Ali Hajizadeh, military aerospace chief said.

The RQ-170 Sentinel was shown on Iranian state television last December.

Tehran says it was brought down using electronic warfare; Washington says it malfunctioned.

US Senator Joe Lieberman dismissed the claim that a copy was being made as "Iranian bluster" saying, "they're on the defensive because of our economic sanctions against them".

But Gen Hajizadeh said: "Our experts have full understanding of its components and programmes."

He said that Iran had managed to hack the controls of the drone, thus enabling the Iranians to reverse-engineer the aircraft to make its own copy.

US officials have previously asked for the drone to be returned but Iran has refused, saying that the US should instead apologise for invading Iranian air space.

Washington has long said that Iran will find it hard to exploit any data and technology aboard it because of measures taken to limit the intelligence value of drones operating over hostile territory.

Although analysts believe lessons could certainly be learnt about how the machine was put together, reverse-engineering has generally been seen by experts as beyond Iran's capability.