Iran's crude oil exports increased in May after a decline in April, according to sources who track tanker movements, moving above the level allowed by November's interim deal on curbing Tehran's nuclear programme, Reuters reported.
The increase, which appears to be led by higher sales to China in particular, could revive concerns in Washington that a softening of sanctions has given Tehran's economy a bigger boost than planned.
Iran's exports have averaged 1.38 million barrels per day (bpd) in May, one of the sources said. That represents an increase from 1.1 million bpd in April, as estimated by the Paris-based International Energy Agency.
"Overall exports have increased in May," the source said. "We have seen more vessels paying visits to Turkey. China has increased imports this month."
A second source, who also tracks Iranian exports, also noted that loading data pointed to a rise in Iranian shipments in May, including robust shipments to China.
"Exports are up, and China does not seem to be listening to the sanctions," the source said.
China's crude imports from Iran more than doubled in April against the same month last year to a record of nearly 800,000 bpd, Chinese customs data showed last week.
Under an interim deal signed in November between Iran and six world powers, which came into effect in January, Iran's exports should average 1 million bpd through to July 20.
Though higher exports since late 2013 have bolstered Iran's coffers, officials in U.S. President Barack Obama's administration have said they expect Iran's oil sales to fall in the coming months and average 1 million bpd over the entire six-month period under the agreed deal known as the Joint Plan of Action (JPOA).
"More Iranian crude appears to be finding its way into markets in violation of the Joint Plan of Action despite the Obama administration's commitment to Congress that it would keep a tight control over the oil lifeblood of the Iranian economy," said Mark Dubowitz, of U.S.-based independent think-tank Foundation for Defense of Democracies.
"If this continues, it would be a clear violation of Iran's obligations under the JPOA, which binds Tehran to both nuclear and sanctions-related commitments."
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. Consumer countries, meanwhile, detail imports with a time lag or not at all.
Iran's oil minister Bijan Zanganeh said this month that, despite being under Western sanctions, Tehran would increase oil output and exports this year and next.
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