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 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.

North Korea tested two ICBMs in July that could fly about 10,000 km (6,200 miles), putting many parts of the U.S. mainland within range and prompting a new round of international sanctions against Pyongyang.

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.

U.S. President Donald Trump held calls with foreign leaders on Monday, including South Korean President Moon Jae-in and German Chancellor Angela Merkel, and the White House declared that "all options to address the North Korean threat are on the table".

South Korea is talking to Washington about deploying aircraft carriers and strategic bombers to the Korean peninsula, and has been ramping up its own defenses in the meantime.