Iran's oil exports fell in April for a second month, according to sources who track tanker movements, moving closer to levels allowed by November's interim deal on curbing Tehran's nuclear program.
The decline may reflect seasonally lower crude oil demand and U.S. pressure on some customers take less. Signs of higher Iranian sales since late 2013 have led to concern in Washington that a softening of sanctions has given Tehran's economy a boost.
"It looks like India has imported less in April - that could be due to a combination of more pressure from the United States and also lower crude demand at this time of year by Indian buyers," a tanker-tracking source said.
Iran's crude exports have averaged 1.1 million barrels per day (bpd) in April, said one of the sources, down from almost 1.3 million bpd in March. That would bring exports back down to the average 2013 level of Iranian imports.
Under an interim deal signed in November between Iran and six world powers - known as the Joint Plan of Action (JPOA) - that came into effect on Jan. 20, Iran's exports are supposed to be held to an average 1 million bpd through July 20.
Tim Wilson, of U.S. based think tank Foundation for Defense of Democracies (FDD) which backs tougher measures, said exports are still on course to exceed that level over the six-month period because of sharply higher sales in earlier months.
"Under that plan Iran was expected to maintain crude oil exports at the same levels as during 2013 with the State Department monitoring the aggregate over time," said Wilson.
"The State Department should now be having considerable concerns."
The Obama administration believes that Iran's oil sales will fall in coming months and average 1 million bpd over the entire six-month period.
Wilson of the FDD said, "In order to remain within the expected oil export figures, Iran will need to reduce their exports by almost 80 percent for the remainder of the JPOA period."
Besides seasonally lower demand, exports are also down as Iran has sent less crude to its ally Syria - one of the countries Washington does not sanction for taking Iranian oil - the tanker tracking source said.
A second tanker-tracking source said Iran's crude exports in April have shown a downward trend to below 1.2 million bpd from March's rate of 1.3 million bpd.
The sailings in April appear to be higher than those indicated by ship loading data seen by Reuters on April 17, which pointed towards crude exports of just 950,000 bpd.
The November deal, say industry sources, improved sentiment around buying Iranian oil by reducing political risk. The rising trend in exports since then alarmed backers of tough sanctions, who said Iran looked to be getting more relief than Washington had said it would under the agreement.
There is no generally accepted figure for the oil exports of many producers, including Iran. Information takes time to filter out from opaque oil and shipping markets, and consumer countries detail imports with a time lag or not at all.
The West's oil watchdog, the International Energy Agency, provides closely watched Iranian export figures and said global crude imports from Iran reached 1.65 million bpd in February, the highest since June 2012.
So far, the IEA has only published a preliminary March exports estimate of 1.05 million bpd, which it says will probably be revised up.
(Reporting by Jonathan Saul and Alex Lawler; Editing by Veronica Brown and William Hardy)