WASHINGTON - A tsunami warning was issued for New Zealand and other small Pacific islands after a major 7.9 magnitude quake struck in the ocean off American Samoa, U.S. government agencies said on Tuesday.

The Pacific Tsunami Warning Center, a branch of the U.S. National Oceanic and Atmospheric Administration, issued the warning after the quake hit about 120 miles southwest of the remote Pacific island of American Samoa. It struck at a depth of 20.5 miles.

A tsunami watch was in effect for Hawaii, Papua New Guinea and other small Pacific islands.

The agency noted that its warning did not mean a tsunami had necessarily been generated by the quake. The warning was issued because a quake of this magnitude had the power to generate a destructive tsunami, it said.

In 2004, a quake in the Indian Ocean generated a powerful tsunami that killed tens of thousands people in Asia.

(Writing by Stacey Joyce, Editing by Frances Kerry)