WASHINGTON, March 6 (Reuters) - U.S. Defense Secretary Jim Mattis and Japanese Defense Minister Tomomi Inada conferred on North Korea's missile launches in a telephone call on Monday, the Pentagon said, adding both leaders agreed the launches were "unacceptable and irresponsible."

"Secretary Mattis reaffirmed that the United States remains steadfast in its defense commitments to Japan including its extended deterrence commitment," Pentagon spokesman Navy Captain Jeff Davis said in a statement.

North Korea fired four ballistic missiles into the sea off Japan's northwest coast on Monday, angering South Korea and Japan, days after it promised retaliation over U.S.-South Korea military drills it sees as preparation for war.

(Reporting by Phil Stewart; Editing by Sandra Maler)