Disaster management minister Hachiro Okonogi said there are people buried under a collapsed building and authorities are working to confirm the details.
The Tokaido Shinkansen services have been suspended between Maibara and Shin-Osaka stations and the Sanyo Shinkansen Line services have been stopped between Shin-Osaka and Okayama stations.
Water floods out from crack in the road, following an quake in Takatsuki, Osaka, Monday.
Private broadcaster TV Asahi showed firefighters responding to a blaze that broke out after the quake at a house north of Osaka city.
The magnitude 6.1 quake struck the city of Osaka and the surrounding area Monday about 8 a.m.
The magnitude-6.1earthquake, initially measured at 5.9, struck the region, 400 kilometres west of Tokyo, during the morning commute.
According to the JMA, this is the first time that the Osaka prefecture has seen an natural disaster of this intensity since 1923.
No tsunami warning was issued.
Chief Cabinet Secretary Yoshihide Suga told an emergency news conference that the government had received no reports of major damage due to the quake.
Osaka Gas Co said it suspended gas supply to 108,000 households in Osaka Prefecture following the quake, while more than 170,000 homes in Osaka and neighboring Hyogo Prefecture are suffering a blackout, Kansai Electric Power Co said.
The Nuclear Regulation Authority said it had detected no problems at any of the region's atomic power plants. An 84-year-old man in nearby Ibaraki died after a bookshelf fell on top of him at home, according to city officials.
Both Japan and Guatemala are situated on the Ring of Fire, an area of intense seismic and volcanic activity on both sides of the Pacific Ocean.
The quake hit western Japan early Monday, but there were no immediate reports of major damage or risk of tsunami waves, officials said. At 13 kilometers (8.1 miles) it was relatively shallow, and caused heavy shaking that registered a lower 6 on the Japanese scale of 7. "I have experienced a few minor quakes before in Osaka, but this was the first time I felt one as big as this", he said, adding that he usually visits Osaka every six months or so.