Beijing-based trading officials cautioned against reading too much into one month′s figures, which they said may have been skewed due to technical problems such as port congestion that pushed back or forth imported cargoes for customs clearance.

Iran, the world′s fifth-largest crude exporter, shipped into China 3.088 million tonnes of crude, or 727,000 barrels per day last month, a rise of 88 percent from a year ago.

Exports from the world′s top exporter, Saudi Arabia, however, slipped 15.5 percent on year to 2.76 million tonnes, or 650,000 bpd, official data from the General Administration of Customs showed.

"Saudis can′t supply the amount of heavy grades Chinese refineries asked for, as most of the OPEC-agreed cuts are on these heavy grades," said one trader close to state-run Saudi Aramco.

"Chinese may have sorted it out by taking more such grades from Iran."

But the trader said cuts on heavier grades to China - Arab Medium and Arab Heavy - started at the beginning of the year, suggesting the sudden fall in May could be just a blip.

An official with the National Iranian Oil Co said he was not aware of any additional sales that Iran had made above its contractual volumes.

Chinese refiner Sinopec Corp and PetroChina, with many of their plants upgraded in recent years to process more heavy, sour and thus cheaper crude, have become increasingly picky in crude slates, motivated by a new fuel pricing mechanism since January 2009 that more or less guarantees a fixed margin.

Iran′s oil output and exports were not affected by protests after the June 12 presidential election, Iran′s OPEC governor said last week.