(Adds details on upcoming anniversary, warhead payloads, trade, comment from S.Korean foreign minister, paragraphs 6, 11-13, 16-17, 22-23, 26; changes dateline to SEOUL)
* Seoul says scrapping warhead weight limit will help respond to N.Korea threat
* South Korean navy conducts fresh drill
* N.Korea seen moving possible ICBM towards west coast - media
* Haley says North Korea is "begging for war"
SEOUL, Sept 5 (Reuters) - South Korea said on Tuesday 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 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.
South Korea's Asia Business Daily, citing an unidentified source, reported on Tuesday that North Korea had been observed 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 report.
Analysts and South Korean policymakers believe North Korea may engage in another provocation on or around Sept. 9, when the North celebrates its founding day. North Koreas fifth nuclear test fell on the same day last year, reflecting Pyongyangs preference to conduct weapons tests on key holidays for strategic impact.
South Korea is talking to Washington about deploying aircraft carriers and strategic bombers to the Korean peninsula, and has been ramping up its own defences in the meantime.
South Korea's navy held more drills on Tuesday.
"Today's training is being held to prepare for maritime North Korean provocations, inspect our navy's readiness and to reaffirm our will to punish the enemy," an unidentified South Korean naval officer told a Defence Ministry briefing.
U.S. President Donald Trump and his South Korean counterpart Moon Jae-in agreed on Monday to scrap a warhead weight limit on South Korea's missiles, South Korea's presidential office said, enabling it 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.
The United States and South Korea signed a pact in 1979, a year after the South successfully tested a ballistic missile, with Washington expressing the need for limits on ballistic missile capability over concerns that such tests could harm regional security.
South Korea and the United States are technically still at war with North Korea after the 1950-53 Korean conflict ended with a truce, not a peace treaty.
Both sides have thousands of rockets and artillery pieces aimed at each other across the world's most heavily armed border but the North's rapid development of nuclear weapons and missiles has altered the balance, requiring a stronger response from South Korea, officials say.
"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.
Under the current guidelines, last changed in 2012, South Korea can develop missiles up to a range of 800 km (500 miles) with a maximum payload of 500 kg (1,102 pounds).
Most of North Korea's missiles are designed to carry payloads of 100-1,000 kg (220-2,205 pounds), according to Nuclear Threat Initiative (NTI), a U.S.-based think thank.
The Hwasong 14 ICBM, tested twice by the North in July, has a potential range of up to 10,000 km (6,210 miles) and is capable of carrying a 300-700 kg (660-1,540 pounds) warhead, according to the NTI.
"PATIENCE NOT UNLIMITED"
On Monday, U.S. Ambassador to the United Nations Nikki Haley said North Korean leader Kim Jong Un was "begging for war" and urged the 15-member U.N. 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 Moon and German Chancellor Angela Merkel, and the White House declared that "all options to address the North Korean threat are on the table".
Haley said the United States 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.
South Korean Foreign Minister Kang Kyung-wha said on Tuesday she felt her Chinese counterpart, Wang Yi, was open to additional sanctions on North Korea after they discussed the North's sixth nuclear test.
"I cannot tell you exact details as the minister asked me not to disclose the content of our discussion, but I could sense that China could be open to more sanctions," Kang told lawmakers in parliament after her phone call with Wang on Monday.
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 $3 billion annual export revenue by a third by banning exports of coal, iron, lead and seafood.
China accounted for 92 percent of North Korea's trade in 2016, according to South Koreas government trade promotion agency.
(Reporting by Christine Kim; Additional reporting by Michelle Nichols at the UNITED NATIONS and Tim Ahmann and David Shepardson in WASHINGTON; Writing by Lincoln Feast; Editing by Paul Tait)