Fox News is reporting that U.S. Sen. Bob Menendez, already the subject of an ethics inquiry for flights on a donor's private plane, took an unreported flight on the jet in 2008.

Citing anonymous sources, Fox reports Menendez flew on donor and friend Salomon Melgen's plane to a charity golf outing in the Dominican Republic.

Menendez has reported flying on Melgen's jet three times.  However, the embattled Senator did not reimburse Melgen as required by Senate rules until earlier this year.  The flights already are the subject of an ethics investigation in the Senate.

Controversy surrounding Menendez's association with Melgen, a Florida eye doctor, continues to swirl as reports have surfaced in recent weeks that Menendez sought to intervene on Melgen's behalf with several government agencies.

In one instance, Menendez reportedly questioned Medicare officials on an audit of Melgen's practice that found the doctor had overbilled the government by nearly $9 million.  In another instance, Menendez reportedly intervened with the Department of Homeland Security on a port security contract a Melgen-owned business holds with the Dominican government.

Menendez has denied that any of his actions were inappropriate and called the delayed reimbursement for the flights an "oversight. "