The reported "harassment" of a US-flag merchant ship by the Russian military has raised concerns that commercial vessels could be drawn into into geopolitical tensions.

DefenseNews reports that a Russian navy ship and a helicopter were involved in the harassment incident against the 6,000-car Green Ridge (built 1998).

The news service, citing an internal report, said the master of the International Shipholding (ISH)-controlled pure car and truck carrier described the encounter as "intense and threatening".