Struggling Venezuelan state oil company PDVSA says it has begun paying $539 million in interest payments on four bonds hours before a grace period expires.

The company, source of 96 percent of the country's foreign revenues, was declared in partial default by global rating agencies earlier this month and in November when it failed to make payments on certain bonds.

It informed bondholders in a statement on Twitter that on Thursday it began "the transfer process" for the payment of PDVSA bonds maturing in 2026, 2024, 2021 and 2035.

The grace period for making those payments runs out between Friday and Sunday.

"We ratify the solvency and solidity of our oil industry, in a struggle against the illegal imperial sanctions," it said.

The company was referring to sanctions imposed by the United States that bar US persons from engaging in transactions involving new Venezuelan debt.