A Tongan police officer in the capital, Nuku'alofa, said there were no immediate reports of damage or a tsunami.
The temblor, classified by the USGS as a "great" quake, struck 95 miles south of Neiafu, Tonga, and 1,340 miles north-northeast of Auckland, New Zealand. It occurred 20 miles beneath the sea floor.
The U.S. National Weather Service warned that a tsunami could strike Fiji as soon as 1:13 p.m. EDT and New Zealand by 2:21 p.m. EDT.
The U.S. Pacific Tsunami Warning Center issued the tsunami warning but said it was not known whether the quake generated a potentially deadly giant wave.