June 11 (Reuters) - Venezuelan PDVSA's U.S. refining unit Citgo Petroleum is increasing efforts to buy crude on the open market to compensate for declining supplies from its parent, which is dealing with a severe tanker backlog that has raised the prospect of a force majeure declaration, traders said on Monday.
From January through April, Citgo was the customer most affected by PDVSA's inability to fulfill its supply contracts due to declining oil output, which this year has fallen to its lowest level in 33 years, according to internal PDVSA data.
U.S. producer ConocoPhillips' actions to seize PDVSA's assets in the Caribbean since May have extended the Venezuelan firm's export delays, leading to a tanker backlog representing nearly a month's worth of shipments from its main terminal.
As of June 11, PDVSA had 23 million barrels worth of crude to be shipped from Jose port and through new ship-to-ship transfers off its Paraguana Refining Center (CRP), according to Reuters vessel tracking and trade flows data.
Reporting by Marianna Parraga Editing by Chizu Nomiyama