HOUSTON (Reuters) - At least two vessels carrying Venezuelan crude chartered by Chevron ( CVX ) are now stalled in Venezuelan waters due to state oil company PDVSA canceling their export authorizations after the U.S. imposed secondary tariffs on the OPEC member's buyers, two sources with knowledge of the matter said on Friday.

The U.S. oil producer awaits customs paperwork to return the cargoes to ports after PDVSA on Thursday canceled set-sail authorizations to two of the Chevron ( CVX )-chartered vessels that had finished loading. It also suspended a loading permit to a third tanker, the sources said.

Under an individual license to operate in U.S.-sanctioned Venezuela, Chevron ( CVX ) was authorized by the U.S. Treasury Department in March to ship Venezuelan crude cargoes to the United States until late May before suspending operations and exports.

But PDVSA's orders might cut the deadline short.

As of Friday, the Chevron ( CVX )-chartered vessels Dubai Attraction and Carina Voyager remained loaded in Venezuelan waters waiting for paperwork for the cargoes' return, according to the sources and LSEG shipping data.

The cargoes had been declared as exports to Venezuela's customs authority, so Chevron ( CVX ) must now obtain authorization for their return as instructed by PDVSA, the sources said.

Carina Voyager was bound for Chevron's ( CVX ) Pascagoula refinery in Mississippi, while Dubai Attraction was scheduled to transfer its cargo to a Valero Energy ( VLO )-chartered vessel off Aruba, according to the data and sources.

A third vessel, the Pegasus Star, was suspended a window to load this week, the sources added.

Chevron ( CVX ) and PDVSA did not reply to requests for comment. Valero did not immediately respond to a comment request.

It was not immediately clear if the cargoes would be rescheduled by PDVSA.

Chevron ( CVX ), whose joint ventures with PDVSA produce about a quarter of the country's oil output, exported some 250,000 barrels per day (bpd) of Venezuelan crude to the U.S. in the first quarter under its license, granted in 2022.

That license and others to European and Asian companies in Venezuela were canceled by U.S. President Donald Trump's administration last month as Washington accused President Nicolas Maduro of not doing enough to curb illegal migration to the U.S. and restore democracy in Venezuela.

Maduro's government rejects the U.S. sanctions on the country's oil industry, in effect since 2019, saying they amount to an "economic war."