North Korea is ready to conduct another nuclear test at any time, South Korea's defence ministry said Monday, just days after Pyongyang sparked worldwide condemnation with its fifth and most powerful test.

The North has said the test has realised its goal of being able to fit a miniaturised warhead on a rocket, prompting urgent talks at the UN Security Council and calls for new sanctions.

"An additional test could be conducted in a tunnel that branches off from the second tunnel or in the third tunnel, where preparations have been completed," said ministry spokesman Moon Sang-Gyun.

He said the South's military is on full combat-readiness to respond to "further nuclear tests, ballistic missile launches or land provocation".

All five nuclear tests have been conducted at the Punggye-ri site in the country's northeast. The initial one in October 2006 was in the first tunnel and the last four in the second tunnel, according to Seoul's defence ministry.

In a statement hailing the "success" of its test on Friday, the North vowed to take further measures to increase its nuclear strike force "in quality and in quantity".

The yield from Friday's test was estimated at 10 kilotons, almost twice as much as the one Pyongyang conducted only eight months earlier, and analyst say it signals its nuclear capability has grown significantly.

However, opinion is split over whether Pyongyang really has made nuclear warheads small enough to arm its missile units, and even if so what the range of such a nuclear-tipped missile would be.

The ruling party newspaper Rodong Sinmun nevertheless declared Monday that the nuclear programme's "miraculous successes" mean the North has not only US bases in the Asia-Pacific but also the US mainland "in its clutches".

Should Washington launch a war against it, Pyongyang would "blow up the land of America and thus finally root out the source of war on the earth", it said.

The United Nations Security Council has agreed to start work on new punitive measures, even though five sets of UN sanctions since the first test have failed to halt the North's nuclear drive.

At a meeting with ruling and opposition party leaders Monday, South Korean President Park Geun-Hye called for political unity in the face of the mounting threat.

"I believe it is important to show the people that we are moving forward as one mind," she said.

And Japanese Prime Minister Shinzo Abe told military officials that Tokyo was confronting an "extremely harsh reality" with both Pyongyang and Beijing posing serious security challenges to Japan.

He said Pyongyang's decision to conduct two nuclear tests in a span of nine months was "absolutely unacceptable".

-- Strong measures --

Sung Kim, the US State Department's special representative for North Korea policy, said Sunday during a visit to Japan that Washington and Tokyo would work closely to come up with the strongest possible measures.

He also suggested the US may launch its own sanctions.

The envoy will arrive in Seoul later Monday and hold talks with his South Korean counterpart Kim Hong-Kyun the following day.

Washington also plans to stage an overflight of South Korea by two B-1B Lancer supersonic bombers as a show of strength, Yonhap news agency said.

US Forces Korea (USFK) said the flight had been delayed a day because of bad weather and would arrive tomorrow, identifying the planes only as "strategic aircraft".

"The mission is aimed at reinforcing the US commitment to its key ally following North Korea's nuclear test," a USFK spokesman told AFP.

Washington took similar military actions following previous atomic tests.

US troops have been based in South Korea since the 1950-53 Korean War ended only in an armistice and not a full peace treaty. They currently number 28,500.