North Korea may be preparing for a fourth nuclear test, South Korean officials say.

South Korean Unification Minister Ryoo Kihl-jae told lawmakers there are "signs" of increased activity in the North's main nuclear test site.

A newspaper also quoted a South Korean official confirming activity similar to the North's third test on 12 February.

This comes amid North Korea's stepped up rhetoric in recent weeks against the US and South Korea.

Mr Ryoo did not elaborate on the specific intelligence.

But when asked about a news report that North Korea had stepped up activity at its underground atomic test site, he said "there are such signs".

Newspaper JoongAng Ilbo quoted an unnamed South Korean official as saying that South Korean intelligence has detected "increased activity of labour forces and vehicles" at the Punggye-ri nuclear test site in the country's north-east.

"We are closely monitoring the ongoing situation, which is very similar to the situation ahead of the third nuclear test," the official told the newspaper.

"We are trying to figure out whether it is a genuine preparation for a nuclear test or just a ploy to heap more pressure on us and the US."

Embassy warning

The United Nations imposed tough sanctions following North Korea's third nuclear test. Pyongyang responded by stepping up angry rhetoric, including threats to use nuclear weapons and restarting its nuclear reactor.

In recent weeks, the North has shut down an emergency military hotline between Seoul and Pyongyang and stopped South Koreans from working at a joint industrial complex in the North.

On Friday, North Korea warned it would not be able to guarantee the safety of foreign embassy staff in the event of a war.

But some analysts have suggested that the rhetoric is in large part designed to shore up the standing of a young, inexperienced leader, Kim Jong-un, in the eyes of his own people, says the BBC's John Sudworth in Seoul.

On Sunday, the schedule on North Korea's state radio showed a continuing diet of war and retribution, with programmes about biochemical war, nuclear war and military preparations dominating the listing, our correspondent adds.

Meanwhile, Japan's defence ministry said the country's armed forces have been ordered to shoot down any North Korean missile headed towards its territory.

- BBC