South Korea says prepared for another North nuclear test

South Korea is fully prepared for North Korea to carry out a fifth nuclear test in defiance of tough UN sanctions imposed over its surprise fourth test in January, officials said Monday.

Military tensions have escalated sharply on the Korean peninsula in recent weeks, with North Korean leader Kim Jong-Un threatening further tests and missile launches, partly in response to ongoing South Korea-US military exercises.

Last Friday, the North test-fired two medium range ballistic missiles, while a US think-tank said recent satellite images showed activity at the North's main nuclear test site that suggest a further underground detonation could take place "any time".

The South's Unification Ministry, which is responsible for inter-Korean affairs, said it had reached a similar conclusion.

"We believe that a fifth nuclear test can take place right away. The government is preparing for all possibilities," said ministry spokesman Jeong Joon-Hee.

The Defence Ministry also reiterated its previous assessment that the North was ready to carry out another test as soon as the order was made by the leadership.

"In this regard, our military, along with intelligence authorities in South Korea and the United States, are thoroughly monitoring such movements," said ministry spokesman Moon Sang-Gyun.

The North's nuclear test on January 6 was followed the next month by a satellite rocket launch that was widely seen as a disguised ballistic missile test.

The UN Security Council responded with the toughest sanctions imposed to date over the North's nuclear programme, while Seoul and Washington pushed ahead with their annual military exercises.

Since the drills began, Pyongyang has delivered a near daily barrage of angry warnings, including threats of pre-emptive nuclear strikes against the South and the US mainland.

The missiles launched on Friday were the North's first medium-range tests for two years.

They came days after Kim Jong-Un announced that a nuclear warhead explosion test and firings of several kinds of ballistic missiles would be carried out "in a short time".

In a meeting with senior aides on Monday, South Korean President Park Geun-Hye said this was a "very crucial time" for the Korean peninsula.

"Even after the international community adopted strong sanctions, North Korea continues to attempt reckless provocations as shown through Kim Jong-Un's recent order," Park said.