LONDON - Iran's crude oil exports have dropped in April to their lowest daily level this year, tanker data showed and industry sources said, suggesting buyers are curbing purchases before Washington clamps down further on Iranian shipments as expected next month.

The United States reimposed sanctions on Iran in November after pulling out of a 2015 nuclear accord between Tehran and six world powers. Those sanctions have already more than halved Iranian oil exports, the country's main source of revenue.

Shipments are averaging below 1 million barrels per day (bpd) so far this month, according to Refinitiv Eikon data and two other companies that track such exports and declined to be identified. That's lower than at least 1.1 million bpd as estimated for March.

The latest drop deepens supply losses resulting from an OPEC-led global agreement to cut oil production and U.S. sanctions on another OPEC member, Venezuela. Supported by those moves, oil prices have risen 30% this year to $71 a barrel.

"Collapsing Venezuelan oil output and sanctioned Iranian exports have put a big question mark over supply," Norbert Ruecker of Swiss bank Julius Baer said.

While exports could rise later in the month, the drop so far suggests Washington is making progress towards its goal of curtailing shipments to below 1 million bpd from May.

The United States, seeking to avoid an increase in oil prices, granted sanctions waivers to China, India, Greece, Italy, Taiwan, Japan, Turkey and South Korea that allowed them to keep buying some Iranian crude. Those exemptions expire in May and analysts expect a new round to be less generous.

The U.S. government is considering more sanctions against Iran and has the ability not to give waivers at all, a senior Trump administration official said this month.

"We think there are very high chances that China and India and perhaps Turkey will receive (fresh) waivers, but with further cuts," said Sara Vakhshouri of energy consultant SVB Energy International.

There is no definitive figure on how much oil Iran exported in March. Shipments have become more opaque since sanctions returned, and Iran no longer reports its production figures to the Organization of the Petroleum Exporting Countries.

One of the two companies that tracks shipments estimated Iran exported 1.1 million bpd of crude last month, while the other company put the number at 1.3 million bpd. Kpler, another company that tracks Iranian exports, estimated March shipments of crude and condensate at 1.29 million bpd.

Still, there is general agreement that crude shipments have dropped from at least 2.5 million bpd in April 2018, the month before U.S. President Donald Trump withdrew the United States from the nuclear deal with Iran.

Tehran has vowed to keep exporting oil despite U.S. efforts to reduce its shipments eventually to zero.