Japan has dispatched its biggest warship to protect a US military vessel in the Pacific Ocean for the first time in modern history following the enactment of a controversial new security law.

The Izumo, Japan's biggest warship since World War Two, left Yokosuka port in Kanagawa prefecture on Monday with the reported task of protecting a US supply ship within Japanese waters.

The mission is the first operation of its kind since new security legislation championed by the prime minister Shinzo Abe was enacted last year which allowed the nation to expand the role of its troops overseas.