HOUSTON / MEXICO CITY (Reuters) - A fleet loaded with some 7 million barrels of Venezuelan crude has been formed in the Gulf of Mexico, some with supplies purchased before the most recent US sanctions against Venezuela and others whose buyers evaluate who to pay, according to operators and Eikon de Refinitiv data.

Last week's decision by Donald Trump's government to impose sanctions on Venezuela sought to undermine support for President Nicolás Maduro by targeting the country's oil exports to the United States, the source of most of its revenues from abroad.

The sanctions seek to prevent US refineries from paying into PDVSA accounts controlled by the Maduro government - one of the reasons why many oil tankers wait in a limbo before Venezuela without payments being clear. The United States buys about 500,000 barrels of Venezuelan crude per day.

US customers of Venezuela's state-owned PDVSA are required under sanctions to deposit payments in guarantee accounts that have not yet been created. The funds will be controlled by the leader of the National Assembly of Venezuela, Juan Guaidó, who has been recognized by the United States, the European Union and much of Latin America as the country's internal president.

The United States Department of the Treasury and the White House did not respond to a request for comment.

There were more than a dozen oil tankers anchored this week in the Gulf of Mexico or outside Venezuelan waters, according to data from Eikon de Refinitiv, because shipping companies expect payment and instructions from buyers.

Operators said that some of the cargoes were used as floating storage by buyers who took advantage of PDVSA's open market sales before the sanctions. Others were retained by commercial firms that have trouble finding refineries willing to accept oil because of payment difficulties associated with sanctions.

The vessels had been contracted by regular buyers of Venezuelan oil in the United States, including Chevron Corp, the PDVSA refining unit Citgo Petroleum [PDVSAC.UL] and Valero Energy, and brokerages that sell to refineries.