Iran's crude oil exports have surged to their highest in 20 months, far exceeding a one million barrel-per-day limit set by the West under an interim deal on curbing Tehran's nuclear programme.
The International Energy Agency's monthly report revised February's global crude imports from Iran upwards by 240,000 bpd to 1.65 million barrels per day, the highest since June 2012.
Under a landmark deal signed in November between Iran and world powers - known as the P5+1 - that came into effect in January of this year, Iran's exports are supposed to be held at an average 1 million bpd for the six months to July 20.
Tough international sanctions over the past two years have cut Iran's oil exports around a half.
"The question is whether they are going to continue to test the sanctions," Antoine Halff, head of the IEA's oil industry and markets division, told Reuters.
China accounted for 168,000 bpd of the rise in imports in February, India for 93,000 bpd and South Korea for 83,000 bpd.
On the other hand, Japanese imports of Iranian oil were revised lower by 103,000 bpd, according to the IEA.
"Imports of Iranian oil are running well above 2013 levels for the third consecutive month," the report said.
Preliminary data for March show imports from Iran dropped to 1.05 million bpd "but that figure will likely be revised upwards closer to February levels upon receipt of more complete data," it said.
Importers of Iranian crude in March included Albania and Syria in addition to regular buyers China, India, South Korea, Japan and Turkey, the IEA said.
The IEA data is based on statistics provided to the organisation by member and non-member states as well as shipping data.
Tanker tracking sources estimated Iran's crude exports averaged around 1.3 million bpd in March - the fifth straight month they had risen above 1 million bpd.
They said the volume of Iranian oil held on tankers at sea or anchorages was starting to drop after several months, falling to between 9 to 13 Very Large Crude Carrier (VLCC) supertankers - each able to hold a maximum of 2 million barrels of oil - from 16 VLCCs in February and a peak of over 20 VLCCs in 2011.
Tanker broker E.A. Gibson said last week floating storage had fallen to 10 VLCCs - its lowest level since December 2012.
"Also, up until recently, several VLCCs appear to have been storing crude off the Chinese coast but this now appears to be down to just one unit," Gibson said.
Exports of Iranian condensate, a form of very light oil, reached 140,000 bpd in February and 110,000 bpd in March, significantly lower than January's 250,000 bpd, the IEA said.
"The current P5+1 negotiations track provides economic growth potential for Iran. The interim nuclear agreement and sanctions relief is helping to stabilise the economy," said Mark Dubowitz of Foundation for Defense of Democracies, a U.S. think-tank, in a report this week.
"Oil exports have already overshot their expected levels in early 2014 ... As a result, Iranian growth will likely be somewhat higher," said Dubowitz, who supports tougher sanctions.
But shipments to Asia have topped that level at least since November.