TOKYO - A powerful 7.0-magnitude earthquake rocked northern Japan early Saturday, swaying buildings and forcing authorities to close highways and stop high-speed trains. News reports said some people were cut by broken glass.

Authorities said two nuclear power plants in the area were not damaged and continued to operate normally, national broadcaster NHK reported. There was no danger of tsunami.

The 8:43 a.m. quake was centered in the northern prefecture of Iwate about 280 miles north of Tokyo.

The meteorological agency issued a warning of a second quake, and a 5.6-magnitude aftershock hit the same area, but it was unclear whether the warning preceded the aftershock. Japan is experimenting with an earthquake warning system.

Footage from the closest large city, Sendai, showed the force of the quake shook surveillance cameras for 30 seconds. NHK interviewed an official from Miyagi prefecture, where Sendai is located, who said he saw tiles coming off the roofs of some homes.

"It was scary. It was difficult to stand up," said Sachiko Sugihara, a convenience store worker in the town of Oshushi in Iwate prefecture in a separate interview with NHK. "The TV fell over and the refrigerator shook."

Windows broke at a nursery school in the area and NHK said some teachers and children were injured, though it was unclear how seriously. Fire officials said they'd received reports of fallen rocks blocking roads and elevators stopping.

Sendai appeared unscathed.

"So far we have not received any reports of damage or injuries. Everything is normal," said Hideki Hara, a police official in Sendai. "Phone lines, water and electricity are all working right now."

Japan is one of the most earthquake-prone areas in the world. The most recent major quake in Japan killed more than 6,400 people in the city of Kobe in January 1995.