Ratings agency Standard & Poor's (S&P) has downgraded US tanker owner Overseas Shipholding Group (OSG) on concerns over its refinancing.

The corporate credit rating has been cut from B to B-, it said, while the rating for its asset-backed lending (ABL) facility and term loan has been moved to B+'from BB- and the unsecured notes to B- from B.

This downgrade reflects the "upcoming maturities of the company's ABL and term loan in 2019," it said.