By Ju-min Park and Jack Kim
SEOUL (Reuters) - North Korea is ready to conduct an additional nuclear test at any time, South Korea's Defence Ministry said on Monday, three days after Pyongyang's fifth test drew widespread condemnation.
North Korea set off its most powerful nuclear blast to date on Friday, saying it had mastered the ability to mount a warhead on a ballistic missile and ratcheting up a threat that its rivals and the United Nations have been powerless to contain.
"Assessment by South Korean and U.S. intelligence is that the North is always ready for an additional nuclear test in the Punggye-ri area," the site of all five nuclear explosions, South Korean Defence Ministry spokesman Moon Sang-gyun told a news briefing.
"North Korea has a tunnel where it can conduct an additional nuclear test," Moon said.
On Monday, Siegfried Hecker, a former director of the Los Alamos National Laboratory in the United States and a leading expert on North Korea's nuclear program, said Pyongyang's claims that it had standardized a nuclear warhead for mounting on ballistic missiles and could produce as many bombs as it wanted had to be taken seriously.
Writing on the 38 North website which monitors North Korea, Hecker estimated the country had stockpiled sufficient plutonium and highly enriched uranium for approximately 20 bombs by the end of the year and had the capacity to add about seven more a year.
"Its ability to field an ICBM fitted with a nuclear warhead capable of reaching the United States is still a long way off - perhaps 5 to 10 years - but likely doable if the program is unconstrained," Hecker added.
The United States and South Korea are pushing for more sanctions against Pyongyang to close loopholes left in a United Nations Security Council resolution adopted in March.
U.S. Treasury Secretary Jack Lew said on Monday the United States would do all it could to oppose North Korea's nuclear weapons policy with financial tools as coordinated economic sanctions had been shown to work.
"North Korea is an enormous challenge and we will do everything we can to keep the pressure on them. We will continue to sharpen financial tools as we can," he said, while adding that China's participation was very important.
Both China and Russia backed sanctions imposed in March following North Korea's January nuclear test, but their apparent ambivalence about fresh sanctions has cast doubt on the Security Council's ability to quickly form a consensus.
"We expect that China, as one of the Security Council member states, should take this issue seriously and play a very constructive role to come up with a very effective and strong sanctions resolution," a South Korean foreign ministry official said.
The Security Council has denounced the latest test and said it would begin work immediately on a resolution. The United States, Britain and France - three of the five veto-wielding permanent members - have pushed for the 15-member body to impose new sanctions.
However, China's foreign ministry spokeswoman Hua Chunying said sanctions alone could not solve the North Korean nuclear issue. The crux of the issue lay with the United States, not China, she added, in an apparent reference to Washington's refusal to resume negotiations with North Korea until it commits to de-nuclearization.
On Saturday, Russian Foreign Minister Sergei Lavrov said a "creative" response was needed.
Speaking to Lavrov on Monday, Chinese Foreign Minister Wang Yi said China "strongly urged North Korea and other relevant parties to remain calm and exercise restraint, and not take any new steps to intensify tensions", China's Foreign Ministry said.
Russia said Lavrov and Wang condemned North Korea's latest nuclear test in a phone conversation on Monday. Russia and China are the other veto powers on the Security Council.
South Korea's President Park Geun-hye said North Korea's nuclear weapons and missiles posed an "imminent threat."
"As North Korea has publicly said nuclear warheads have been standardized and customized to mount on ballistic missiles, we should keep in mind that North Korea's nuclear missiles are a realistic, imminent threat targeting us, not a simple threat for negotiations," Park told a meeting with major political party leaders.
Pyongyang's assertions that it is able to miniaturize a nuclear warhead have never been independently verified.
BOMBER FLIGHT DELAYED
North Korean Foreign Minister Ri Yong Ho, formerly the country's chief nuclear negotiator, arrived in Beijing on Monday and was seen entering the country's embassy, Japan's Kyodo news agency reported.
Ri left Pyongyang on Monday to attend a meeting of the Non-Aligned Movement countries in Venezuela and later the U.N. General Assembly, North Korea's official KCNA news agency said.
A U.S. special envoy for North Korea, Sung Kim, met Japanese officials on Sunday and said the United States may launch unilateral sanctions against North Korea, echoing comments by U.S. President Barack Obama on Friday in the wake of the test.
South Korea's Yonhap news agency reported that bad weather had delayed the flight of a U.S. B-1B bomber to the Korean peninsula, a show of strength and solidarity with ally Seoul, scheduled for Monday.
The flight from the U.S. base in Guam would now take place on Tuesday, a U.S. Forces in Korea official told Reuters, declining to identify the type of aircraft involved.
A group of 31 conservative lawmakers in South Korea said the country should have nuclear weapons, either by acquiring its own arms, or asking the United States to redeploy tactical nuclear weapons withdrawn under a 1991 pact for denuclearization of the Korean peninsula.
"We should discuss every plan including an independent nuclear armament program at the level of self-defense to safeguard peace," Won Yoo-chul, a senior lawmaker for the ruling Saenuri Party, said in a statement.
South Korea's Defence Ministry said there was no change in its policy barring nuclear weapons.
(Additional reporting by John Ruwitch in Shanghai, Michael Martina in Beijing, David Brunnstrom in Washington and Jonathan Spicer in New York; Editing by Alex Richardson and James Dalgleish)