By Kaori Kaneko and Malcolm Foster
TOKYO, June 19 (Reuters) - A magnitude 6.1 earthquake in Osaka, Japan's second-biggest metropolis, killed four people, injured hundreds more and halted factory lines in an industrial area, government and company officials said on Tuesday.
Authorities were assessing the damage from Monday's quake which injured 380 people, according to the latest government estimate on Tuesday.
Live footage showed toppled walls, broken windows and water gushing from burst mains after the quake hit Osaka, which will host next year's Group of 20 summit, just before 8 a.m. on Monday (2300 GMT Sunday) as commuters were heading to work.
Quakes are common in Japan, part of the seismically active "Ring of Fire" that stretches from the South Pacific through Indonesia and Japan, across to Alaska and down the west coast of North, Central and South America.
The epicentre of Monday's earthquake was just north of Osaka city at a depth of 13 km (8 miles), said the Japan Meteorological Agency. The agency originally put the magnitude at 5.9 but later raised it to 6.1.
Collapsing walls killed a 9-year-old girl as she walked to school and an 80-year-old man, the government said.
An 85-year-old man died when a bookcase fell on him and an 81-year-old woman was found dead under a toppled dresser.
The quake struck a key industrial area of central Japan.
Trade minister Hiroshige Seko said on Tuesday the quake had forced factories to halt operations but there was no information on the damage to facilities.
"We will closely monitor the impacts, including on supply chains," Seko told reporters.
Osaka-based electronics firm Panasonic Corp said it resumed some operations at three of its plants.
Daihatsu Motor Co, a unit of Toyota Motor Corp, stopped day-time operations in its factories in Osaka and Kyoto and at one plant in Shiga while the company checked for damage.
Tractor maker Kubota Corp. said it halted two plants in the area, while air conditioner maker Daikin Industries Ltd suspended operations at two plants, one of which had restarted.
Honda Motor Co. and Mitsubishi Motors said they were resuming operations after suspensions and safety checks. Sharp Corp also restarted work at a joint venture plant with parent Hon Hai Precision Industry that it had stopped for safety checks.
Japan's JXTG Nippon Oil & Energy Corp is likely to take about a week to resume normal refining operations at its 115,000 barrels-per-day Osaka refinery that was shut down following the quake, the trade ministry said.
Tourists spoke of the panic when the earthquake struck.
"We were sleeping and it woke us up abruptly," said Kate Kilpatrick, a 19-year-old American who was staying at a Osaka hotel.
"It was so terrifying because this is my first earthquake. I thought it was a nightmare because I was so confused," she said.
No irregularities were detected at the Mihama, Takahama and Ohi nuclear plants to the north of Osaka, said Kansai Electric Power. It said more than 170,000 households in Osaka and neighbouring Hyogo prefecture lost power for a short time.
Osaka Gas Co said it would take eight to 12 days to resume piped gas supplies to more than 110,000 customers in the quake-hit area.[nL4N1TK212 ]
Most trains in the area had resumed service on Tuesday, Osaka police said.
Osaka prefecture, which includes the city and surrounding areas, is home to 8.8 million people. The city is close to Kobe, which was hit by a deadly magnitude 6.9 quake in 1995.
A massive 9.0 quake struck further north in March 2011, triggering a huge tsunami that killed some 18,000 people and the world's worst nuclear disaster in a quarter of a century at Tokyo Electric Power's Fukushima Daiichi nuclear plant.
Japan introduced a law after the Kobe quake requiring owners of large buildings such as hotels and hospitals to have their buildings inspected for earthquake resistance. (Reporting by Kaori Kaneko, Mari Saito, Kiyoshi Takenaka, Osamu Tsukimori, Makiko Yamazaki, Naomi Tajitsu, Linda Sieg, Ami Miyazaki and Maki Shiraki Writing by Malcolm Foster Editing by Darren Schuettler)
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