Tokyo ? A strong earthquake jolted northwestern Japan on Monday morning and caused buildings in the capital Tokyo to sway. The Meteorological Agency said small tsunamis as high as 20 inches were believed to have hit coasts in the area.

Three nuclear reactors in the region automatically stopped, and an electrical fire was reported at one, public broadcaster NHK said. Black smoke was seen billowing from the reactor as the fire burned.

The quake had a magnitude of 6.7, according to the U.S. Geological Survey. It was centered off the coast of the prefecture (state) of Niigata, the Meteorological Agency said in a statement.

The Meteorological Agency issued tsunami warnings along the coasts of Niigata and warned of small tsunamis along other coasts.

But no significant damage was expected and the warnings were later lifted.

Several bullet train services linking Tokyo to northern Japan were suspended, NHK said.

Kyodo News reported three houses collapsed in Kashiwazaki City in Niigata. Police were searching for anyone who may have been trapped inside, the report said.

Prime Minister Shinzo Abe, who was in Nagasaki in southern Japan campaigning for the parliamentary elections later this month, will return to Tokyo to deal with the quake, Kyodo said.

Niigata Airport, which had suspended flights shortly after the quake, resumed services, Kyodo said.

Japan sits atop four tectonic plates and is one of the world’s most earthquake-prone countries. The last major quake to hit the capital, Tokyo, killed some 142,000 people in 1923, and experts say the capital has a 90 percent chance of suffering a major quake in the next 50 years.

In October 2004, a magnitude-6.8 earthquake hit Niigata, killing 40 people and damaging more than 6,000 homes. It was the deadliest to hit Japan since 1995, when a magnitude-7.2 quake killed 6,433 people in the western city of Kobe.