Iran is concealing the destination of its oil sales by disabling tracking systems aboard its tanker fleet, making it difficult to assess how much crude Tehran is exporting as it seeks to counter Western sanctions aimed at cutting its oil revenues, according to Reuters.

Most of Iran's 39-strong fleet of tankers is now "off-radar" after Tehran ordered captains in the National Iranian Tanker Co (NITC) to switch off the black box transponders that are used in the shipping industry to monitor vessel movements, oil industry, trading and shipping sources said.

"Iran, helped by its customers, is trying to obfuscate as much as possible," said a senior executive at a national oil company that has done business with Iran.

And Iran may have countered a reported reduction in its oil sales in March by offering big discounts in the form of free freight, finance and insurance and generous credit terms, the sources said.

Europe's July 1 oil embargo, and U.S. and European financial sanctions against Iran's nuclear programme have seen Tehran's oil sales drop to most Western destinations and drawn promises from some Asian buyers that they will cut purchases.

But cheap, covert sales may have curbed or even reversed the reduction in shipments, the sources say. Ship captains steering NITC supertankers have switched off recognition systems and customers are keeping business strictly under wraps. "People are being very secretive right now. They are not talking about this on email, Yahoo or mobile," said the head of a crude oil desk at a top oil trading houses.

A Reuters' survey of the Iranian fleet via the ship tracking system AIS (Automatic Identification System) Live shows only seven of its 25 very large crude carriers are still operating on-board transponders, allowing computers to track vessels.

Only two of NITC's nine smaller Suezmax size tankers now have their tracking systems in operation, shipping sources say.