Traders in the London oil markets have reported a sudden big cut in crude oil deliveries from Syria which, they say, could prove the country has been illegally importing Iraqi oil.
This follows a warning to customers on Tuesday that Syria was cutting oil exports by nearly half for the rest of the year, from state oil marketer Sytrol.
The fall is believed to be linked to the bombing of a pipeline from Iraq to Syria by American special forces.
If this proves correct, analysts say it may confirm Syria has been importing Iraqi oil in contravention of UN sanctions for many years.
Axel Bush from Energy Intelligence has spent three years investigating illegal Iraqi oil exports.
He told the BBC's World Business Report that many people involved in the oil markets believe Syria has been importing 200,000 barrels of Iraqi crude every day for years.
It is thought the cut-price Iraqi oil was piped from oil fields near the Southern town of Basra to Syrian refineries.
This allowed Syria to export more of its own oil at the higher international price.
Syria has always maintained that the pipeline in question was being tested but was not in full use.
Mr Bush also said other countries have been the recipients of Iraqi oil exports since sanctions began.
"There's been dribs and drabs going out through Turkey and there's sort of a... nudge and wink understanding with Jordan that they can get 110,000 barrels a day which everybody knows about and nobody's bothered about," he said.
Mr Bush also said at least four large crude oil carriers had left an Iraqi port in the weeks leading up to the war carrying at least four million barrels of oil, but their destination is unknown.
"The multinational interception force which was... supposed to control these things, have done absolutely nothing," he said.
"It is a great mystery, where has that oil gone?"