SEOUL/UNITED NATIONS (REUTERS, AFP) - South Korea said on Tuesday (Sept 5) that an agreement with the United States to scrap a weight limit on its warheads would help it respond to North Korea's nuclear and missile threat after Pyongyang conducted its sixth and largest nuclear test two days ago.

South Korean President Moon Jae In and US President Donald Trump agreed on Monday to scrap a warhead weight limit on South Korea's missiles, South Korea's presidential office said, enabling the country to strike North Korea with greater force in the event of a military conflict. The White House said Trump gave "in-principle approval" to the move.

"We believe the unlimited warhead payload will be useful in responding to North Korea's nuclear and missile threats," Defence Ministry spokesman Moon Sang Gyun told a briefing on Tuesday.

Moon's office acknowledged the possibility of a need to purchase advanced US weapons systems in the future as part of efforts to enhance South Korea's deterrence against North Korean provocations, but dismissed any immediate plan to spend "many billions of dollars" as suggested by Washington, Yonhap news agency reported.

"In their discussions so far, the two leaders have shared a view that there is a need to upgrade South Korea's defence capabilities, including an early establishment of its three-axis defence system, to counter threats from North Korea's nuclear and missile threats, and agreed to continue their cooperation to that end," Moon's spokesman Park Soo Hyun said in a statement, according to the news report.

The three-axis defence system refers to South Korea's own kill chain, missile defence system and massive retaliation capabilities.

Meanwhile, South Korea's Asia Business Daily, citing an unidentified source, reported on Tuesday that North Korea had been spotted moving a rocket that appeared to be an intercontinental ballistic missile (ICBM) towards its west coast.

The rocket started moving on Monday and was spotted moving only at night to avoid surveillance, the report said.

South Korea's defence ministry, which warned on Monday that North Korea was ready to launch an ICBM at any time, said they were unable to confirm the contents of the report.

South Korean officials believe more provocation from the reclusive state is possible, despite international outrage over Sunday's test and calls for more sanctions on North Korea.

TACTICAL US NUCLEAR WEAPONS

The South's defence minister on Monday said it was worth reviewing the redeployment of American tactical nuclear weapons to the Korean Peninsula to guard against the North, a step that analysts warn would sharply increase the risk of an accidental conflict.

Under the current guidelines, last changed in 2012, South Korea can develop missiles up to a range of 800km with a maximum payload of 500kg.

Song Young Moo, the South's defence minister, said that he asked his American counterpart, James Mattis, during talks at the Pentagon last week that strategic assets such as US aircraft carriers, nuclear submarines and B-52 bombers be sent to South Korea more regularly.

"I told him that it would be good for strategic assets to be sent regularly to the Korean Peninsula and that some South Korean lawmakers and media are strongly pushing for tactical nuclear weapons (to be redeployed)," Song told a parliamentary hearing on North Korea's nuclear test, without disclosing Mattis's response.

A poll that YTN, a cable news channel, commissioned in August found that 68 per cent of respondents said they supported bringing tactical nuclear weapons back to South Korea.

"The redeployment of tactical nuclear weapons is an alternative worth a full review," Song said, echoing a position closely associated with conservatives in South Korea but not with progressives like Moon, who was elected president in May after vowing to engage with the North.

The United States had about 100 nuclear-armed weapons, including short-range artillery, stationed in South Korea until 1991. Then President George H.W. Bush signed the Presidential Nuclear Initiatives and withdrew all tactical nuclear weapons that had been deployed abroad.

Shortly afterwards, the two Koreas signed an agreement committing to making the peninsula free of nuclear weapons - a deal that North Korea violated by developing its own nuclear arms. But Pyongyang has maintained that Seoul has also broken its promise because remaining under the US nuclear umbrella is tantamount, it says, to having such weapons.

After the defence minister spoke at the hearing, the South Korean president's office said that it was not considering redeploying tactical nuclear weapons. "Our government's firm stance on the nuclear-free peninsula remains unchanged," said Kim Dong Jo, a spokesman for Moon.

Military experts in the United States are almost universally opposed to the idea of deploying strategic or tactical weapons in South Korea."The thing that most concerns me about redeployment is that it introduces more room for miscalculation or unintended escalation," said Catherine Dill of the Center for Nonproliferation Studies in Monterey, California.

In that situation, the ability to react more quickly could be a negative factor.

South Korea's navy meanwhile held more drills on Tuesday. The drills, conducted in the East Sea, also known as the Sea of Japan, involved the 2,500-tonne Gangwon frigate, a 1,000-tonne patrol ship and 400-tonne guided-missile vessels, among others, the Navy said in a statement.

"If the enemy launches a provocation above water or under water, we will immediately hit back to bury them at sea," Captain Choi Young Chan, commander of the 13th Maritime Battle Group, said in a statement.

North Korea tested two ICBMs in July that could fly about 10,000km, putting many parts of the US mainland within range and prompting a new round of international sanctions against Pyongyang.

'PATIENCE NOT UNLIMITED'

On Monday, US Ambassador to the United Nations Nikki Haley said North Korean leader Kim Jong Un was "begging for war" and urged the 15-member UN Security Council to impose the "strongest possible" sanctions to deter him and shut down his trading partners.

Trump held calls with foreign leaders on Monday, including German Chancellor Angela Merkel, and the White House declared that "all options to address the North Korean threat are on the table".

Speaking at the UN, Haley said the US would circulate a new Security Council resolution on North Korea this week and wanted a vote on it on Monday.

"War is never something the United States wants. We don't want it now. But our country's patience is not unlimited. We will defend our allies and our territory," Haley said.

China, North Korea's main ally and trading partner, and Russia called for a peaceful resolution to the crisis.

"China will never allow chaos and war on the (Korean) peninsula," said Liu Jieyi, the Chinese ambassador to the United Nations, urging North Korea to stop taking actions that were"wrong" and not in its own interests.

Russia said peace in the region was in jeopardy. "Sanctions alone will not help solve the issue," Russia's UN Ambassador Vassily Nebenzia said.

North Korea has been under UN sanctions since 2006 over its ballistic missile and nuclear programmes. Typically, China and Russia only view a test of a long-range missile or a nuclear weapon as a trigger for further possible UN sanctions.

US Defence Secretary James Mattis warned of a "massive" military response if the US or its allies were threatened in the wake of Sunday's test. Pyongyang said it had successfully tested an advanced hydrogen bomb for a long-range missile on Sunday, something experts believe it has now achieved or is very close to achieving.

Trump has previously vowed to stop North Korea developing nuclear weapons and said he would unleash "fire and fury" if it threatened US territory.

Despite the tough talk, the immediate focus of the international response was on tougher economic sanctions.

Diplomats have said the Security Council could now consider banning North Korean textile exports and its national airline, stop supplies of oil to the government and military, prevent North Koreans from working abroad and add top officials to a blacklist to subject them to an asset freeze and travel ban.

The sanctions imposed after July's missile tests aimed to slash Pyongyang's US$3 billion (S$4.1 billion) annual export revenue by a third by banning exports of coal, iron, lead and seafood.