PARIS, France (AFP) — Three of 11 vessels chartered by Chevron were Thursday transporting oil from Venezuela to the United States (US), according to an AFP analysis of ship-tracking data, as export sanctions raised concerns over the South American country’s storage capacity.

Two more of the tankers were anchored at Bajo Grande refinery port, western Venezuela, while the remaining six were on their way to the South American country, according to ship-tracking data collated by Bloomberg and trade information platform Kpler.

Chevron is the only US company to have operations in Venezuela and booked the tankers as part of a regular schedule of crude shipments back to the US.

While the boats were in transit, US President Donald Trump said Tuesday that Caracas will turn over 30 million to 50 million barrels of sanctioned crude to the US.

The US blockade of Venezuelan exports is causing a build up of oil in storage, warned analysts from Kpler.

Chevron is one of the operators still able to move the oil.

One of its chartered tankers, the Ionic Anassa, was last recorded passing Cuba on its way to Pascagoula Port in Mississippi, having loaded at Bajo Grande on January 4, according to shipping data published by Bloomberg and Marine Traffic.

The Nave Photon was recorded north of Caracas on Thursday, heading for Port Freeport in Texas, having arrived at the Jose terminal, eastern Venezuela, on January 5.

It was being closely followed by The Mediterranean Voyager, which also appeared to have picked up a shipment at Jose terminal, according to the latest displacement data published by Bloomberg.

Two vessels, the Minerva Gloria and Searuby, were anchored at Bajo Grande on Thursday. Gloria appeared to be loaded, while Searuby appeared to be empty.

Another six vessels, apparently empty, were en route to the South American country.

Chevron would not confirm the movements of the vessels when asked by AFP, saying only that it “remains focused on the safety and wellbeing of our employees” and that “we continue to operate in full compliance with all relevant laws”.

Crude oil stocks in Venezuela have been rising since the US seizure of the tanker Skipper on December 10, according to data from Kpler.

The latest assessment, based on radar images from December 30, indicates that onshore stocks, namely tanks, have exceeded 22 million barrels, nearly half of the country’s storage capacity.

“With loading activity now slowing sharply due to the naval blockade, Kpler analysts expect upcoming radar imagery to show further inventory builds,” said Emmanuel Belostrino, senior analyst at Kpler, in a report sent to AFP.

“Floating” oil storage is also increasing rapidly off the coast of Venezuela, a sign of a bottleneck in exports.

According to Kpler figures, at least 16.7 million barrels of crude oil were stored on board at least fifteen large tankers, used as temporary reservoirs near the Venezuelan coast, at the beginning of the week.