Tsunami hits Japan's northern coast after a strong quake rocked Fukushima on Tuesday, briefly disrupting cooling functions at a nuclear plant.

A magnitude of 7.4 earthquake was centered off the coast of Fukushima prefecture at a depth of about 10 km (6 miles).

There were no reports of deaths or serious injuries several hours after the quake hit at 5:59 a.m. (2059 GMT Monday).

A tsunami of up to 1.4 meters (4.5 feet) had been observed around Sendai, about 70 km (45 miles) north of Fukushima, with smaller waves hitting ports elsewhere along the coast, public broadcaster NHK said.

All Japan's nuclear power plants on the coast threatened by the tsunami are shut down in the wake of the March 2011 disaster, which knocked out Tokyo Electric Power Co's Fukushima Daiichi nuclear plant, spilling radiation into the air and sea.

No other damage from the quake has been confirmed at any of its power plants, although there have been blackouts in some areas, the spokeswoman said.

Earthquakes are common in Japan, one of the world's most seismicaly active areas. Japan accounts for about 20 percent of the world's earthquakes of magnitude 6 or greater.

On the U.S affairs, the State Department issued an alert to U.S. citizens on Monday about a heightened risk of terrorist attacks throughout Europe, particularly during the holiday season.

"U.S. citizens should exercise caution at holiday festivals, events, and outdoor markets," it said in a statement, adding that there was credible information that Islamic State, al Qaeda and their affiliates continued to plan attacks in Europe.

The department noted extremists had carried out attacks in Belgium, France, Germany and Turkey in the past year, and said it remains concerned about the potential for attacks throughout Europe.

"U.S. citizens should exercise vigilance when attending large holiday events, visiting tourist sites, using public transportation, and frequenting places of worship, restaurants, hotels, etc.," the statement said.

The travel alert expires on Feb. 20, 2017.

Meanwehile, A Tennesse school bus carrying elementary students crashed on Monday afternoon, killing at least six children and sending nearly two dozen to a hospital with injuries.

Five children were found dead inside the vehicle, and a student died at a hospital, said Melydia Clewell, spokeswoman for the Hamilton County District Attorney's office. It was not clear how many students were riding in the bus when it crashed.

The students were in kindergarten through fifth grade, which would make them roughly ages 5 to 10.

The accident left the yellow school bus wrapped around a tree, mangled and nearly severed in two.

Speed appeared to have contributed to the accident, Chattanooga Police Chief Fred Fletcher said.

The bus driver is being questioned and cooperating with investigators, the Chattanooga Police said on Twitter. Authorities are also investigating whether alcohol or drug use may have contributed to the crash.