SEOUL, South Korea — A small earthquake was detected on Saturday afternoon near North Korea’s underground nuclear test site, reviving fears that the country had set off another bomb.

Chinese seismologists said the activity was most likely caused by an explosion, but the United States Geological Survey said it was too early to say whether the cause was natural or artificial.

The South Korean seismology agency said the quake was probably natural, and the North did not immediately issue a statement or say that one was coming, as it has done when conducting previous tests.

The three agencies gave varying estimates for the magnitude of the earthquake: the Americans, 3.5; the Chinese, 3.4; and the South Koreans, 3.0.