SEOUL, SOUTH KOREA: South Korean officials detected an "artificial earthquake" near North Korea's main nuclear testing site on Wednesday, a strong indication that the nuclear-armed country had conducted its fourth atomic test.

The US Geological Survey measured the magnitude of the seismic activity at 5.1 on its website. An official from the Korea Metrological Administration, South Korea's weather agency, said it believed the earthquake was caused artificially, without elaborating, and originated 49 kilometres (30 miles) north of Kilju, the northeastern area where North Korea's main nuclear test site is located. The country conducted all three previous atomic detonations there.

South Korean government officials couldn't immediately confirm whether a nuclear blast or natural earthquake had taken place.

North Korea conducted its third nuclear test in February 2013. A confirmed test would mark another big step toward Pyongyang's goal of building a warhead small enough to be mounted on a missile capable of reaching mainland America's shores.

A test would further North Korea's international isolation by prompting a push for new, tougher sanctions at the United Nations and worsening Pyongyang's already bad ties with Washington and its neighbors.

Pyongyang is thought to have a handful of crude nuclear weapons. The United States and its allies worry about North Korean nuclear tests because each new blast brings the country closer to perfecting its nuclear arsenal.