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KATHMANDU, Sept 25 (Reuters) - An earthquake measuring 5.7 struck western Nepal on Thursday, the United States Geological Survey said.

The quake, centred 400 km (250 miles) northwest of Kathmandu, was at a depth of 10 km, the USGS said. There were no immediate reports of casualties, a Nepali official said.

"It is not an emergency," Somnath Sapkota, chief of the National Seismological Centre in Kathmandu, said, adding it was a small tremor.

(Reporting by Gopal Sharma; Writing by Sanjeev Miglani;

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