TOKYO -- A powerful earthquake struck Hokkaido in northern Japan early Thursday morning, leaving almost three million households without power.

The early morning earthquake caused power outages across Hokkaido, affecting 2.95 million households. Traffic lights are not working in Sapporo, the island's largest city, and it is unclear when the subway and streetcars there will resume service.

The outages were due to thermal power stations across Hokkaido shutting down after the tremor. Hokkaido's three nuclear power plants were already out of service.

The Tomato-Atsuma coal-fired plant, the biggest thermal power station on the island, was damaged by the quake. At a news conference during the morning, Chief Cabinet Secretary Yoshihide Suga said that it would be difficult to resume operations at the facility quickly.

Minister of Economy, Trade and Industry Hiroshige Seko, who earlier said that he had ordered Hokkaido Electric Power Company to restore power "within a few hours," has since told the press that it would "take at least one week to restore coverage for the whole of Hokkaido."

All three nuclear reactors at Hokkaido Electric Power's Tomari plant were offline when the quake hit. The facility is currently not receiving any outside power but cooling pools for its nuclear fuel are still operating safely thanks to backup diesel generators.

Tohoku Electric Power said no abnormalities were detected at the Higashidori nuclear power plant in Aomori Prefecture.

The Idemitsu Kosan oil refinery in the city of Tomokomai suspended operations and shipments, although there was no reported damage to the facility. JXTG Nippon Oil & Energy also stopped shipments from its eight facilities in Hokkaido.

The earthquake hit just two days after a major typhoon tore through Japan's Osaka region, with the natural disasters disrupting economic activity in the north and west of the country.

The temblor, with magnitude of 7, occurred at 3:08 a.m. The epicenter was in the Iburi region in southwestern Hokkaido. Two fatalities and about 120 injuries have been reported. Thirty-one people remain missing in the town of Atsuma.

A fire has broken out at a Mitsubishi Steel Mfg. facility located within Nippon Steel & Sumitomo Metal's steelworks in Muroran, according to the local fire department. There are no reports of injuries.

Paper factories of Nippon Paper Industries and Oji Holdings stopped operation.

The operator of New Chitose Airport, the main gateway to Hokkaido -- the northernmost of Japan's four main islands -- said that the terminal building will be closed Thursday and all flights will be canceled due to damage to the building.

JR East said it has suspended bullet train service between Shin-Aomori Station in Aomori, the northernmost prefecture of Japan's main island, and Shin-Hakodate-Hokuto Station in Hokkaido.

JR Hokkaido has suspended all rail services, as has the Sapporo Municipal Subway.

Several houses and streets were damaged in Sapporo. © Kyodo

"The shaking was so strong that I couldn't walk," said an Atsuma town employee. "A large part of the town appears to be without power."

Hokkaido accounts for a little more than 20% of Japan's total land area, and has a population of roughly 5.4 million.

Hokkaido is a popular tourist destination, famed for its skiing resorts, hot springs, gourmet food and natural parks. In 2017, 2.79 million foreigners visited the prefecture, according to prefecture's tourism bureau. The number has almost quadrupled in 10 years.

About 90% of foreign tourists last year were from Asia. Visitors from China, South Korea, and Taiwan made up large proportions, with more than 600,000 arriving from each.

Tourism contributes about 632 billion yen ($5.6 billion) to Hokkaido's gross domestic product, according to a survey conducted by the bureau in May 2017. The sector accounted for 3.4% of the prefecture's entire economy, more than either finance or agriculture.

The quake shook popular resorts like Niseko and Tomamu.

The Sapporo Securities Exchange ceased operations due to the blackout. It has yet to announce what its plans are for Friday. Over 50 companies are listed on the exchange, including those co-listed on the Tokyo Stock Exchange. Fifteen companies, including the health and fitness Rizap Group, are exclusively listed in Sapporo.

Akane Okutsu and Masayuki Yuda in Tokyo contributed to this report.