Iran is storing millions of barrels of oil on tankers in its territorial waters as Tehran struggles with tougher Western sanctions on its vital seaborne export trade, ship industry sources say.

Iran's oil revenues have fallen by about 50 percent since tough EU and US measures were imposed last year, hurting business and cutting living standards for ordinary Iranians.

"There is no doubt there are more Iranian tankers being used for floating storage at the moment on their side of the Gulf and the feeling is this is expected to rise," said a European ship industry source with knowledge of tanker movements.

"The embargo is hurting and there has been talk of attempts by Iran to unload oil cargoes at distressed prices."

Ship industry sources gave varying estimates on how much oil Iran was storing at sea, but all said the volume was higher.

Data from maritime intelligence publisher IHS Fairplay estimated 10 of Iran's supertankers, each able to carry up to 2 million barrels of crude, were storing oil, together with one smaller tanker able to carry up to 1 million barrels.

The data showed a further two supertankers were also probably deployed on floating storage, based on the length of time they were stationary, meaning as much as 25 million barrels in total.

The vessels, all belonging to Iran's top tanker operator NITC, were located close to the Iranian oil terminals of Assaluyeh, Kharg Island and Bahregan, the data showed.

"There seem to be more vessels than there were four months ago - the big area which seems to have changed is off Assaluyeh," said Richard Hurley, a senior analyst at IHS Fairplay.

"There are more ships that seem to have come in to that anchorage in the past four months or so. At one point they were down to a core storage fleet of around six vessels anchored off Kharg Island and Assaluyeh."

NITC officials could not be reached for comment.

Please LIKE our Facebook page - it makes us stronger