HOUSTON, Dec 15 (Reuters) - A tanker carrying Russian

naphtha for Venezuelan state oil company PDVSA and at least four

supertankers due to pick up crude cargoes in Venezuela have made

u-turns after the U.S. seized a vessel carrying Venezuelan

crude, ship monitoring data showed on Monday.

The U.S. Coast Guard last week intercepted and seized a very

large crude carrier (VLCC) carrying some 1.85 million barrels of

Venezuelan heavy oil sold by PDVSA, a sign of increasing

friction between Venezuela and the U.S., which has ramped up

pressure on President Nicolas Maduro.

The seizure left more than 11 million barrels stuck onboard

other vessels in Venezuelan waters and has prompted some tanker

owners to order u-turns to avoid problems, with an armada of

U.S. ships patrolling the Caribbean Sea.

Benin-flagged tanker Boltaris, which was carrying some

300,000 barrels of Russian naphtha bound for Venezuela, made a

u-turn late last week and is now heading for Europe without

having discharged, according to LSEG vessel monitoring data.

At least four VLCCs that were in PDVSA's schedules to load

crude at Venezuelan ports in the coming weeks have also made

u-turns in recent days, monitoring service TankerTrackers.com

said.

The U.S. pressure has almost paralyzed Venezuela's oil

exports since last week, with only tankers chartered by Chevron ( CVX )

setting sail carrying Venezuelan crude to the U.S. under

an authorization previously granted by Washington, the data

showed.

PDVSA said on Monday it had been hit by a cyberattack, which

according to sources forced the shutdown of the company's

administrative and operational systems on Monday, including its

oil delivery system.