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CARACAS, VENEZUELA (AP) -- Venezuela's state oil company is behind on billions in payments to private oil contractors from Oklahoma to Belarus.

Some contractors have now stopped work, even as Venezuela's President Hugo Chavez funnels more oil revenue to social programs.

Petroleos de Venezuela says unpaid invoices jumped 39 percent in the first nine months of last year -- reaching $7.86 billion in September. And that was when world oil was selling for $100 a barrel.

But with prices plummeting by more than half, the company's trying to renegotiate some contracts.

But analysts say hardball tactics to reduce charges from crucial service providers could backfire by lowering Venezuela's oil output.

 And foreign debt markets are reflecting jitters about Venezuela's finances.

Oil accounts for 94 percent of Venezuela's exports and funds nearly half the socialist government's budget.

Chavez uses it to bankroll an international aid bonanza, showering allies with cheap fuel, refining projects and cash donations.

But Tulsa, Okla.-based contractor Helmerich & Payne said last week that it has stopped drilling with two of its 11 oil rigs in Venezuela because of delayed payments.

Dallas-based Ensco International said it suspended operations on an oil rig off Venezuela's Caribbean coast because it was owed $35 million, prompting PDVSA to take over operations.

(Copyright ©2009 by The Associated Press. All Rights Reserved.)