Reports that North Korea is preparing for another intercontinental missile attack has prompted China to warn against it, saying that North Korea should not worsen tensions.
South Korea has detected signs the North is preparing another missile launch, the defence ministry said on Monday.
The ministry said signs that North Korea was "preparing for another ballistic missile launch have consistently been detected since Sunday's test", referring to Pyongyang's sixth nuclear test.
It did not give details or indicate when a launch might take place, but it was reported it could involve an intercontinental ballistic missile.
Sunday's blast had a strength of 50 kilotons, defence ministry officials told a parliamentary briefing.
Pyongyang on Sunday triggered global alarm with by far its most powerful test to date after it claimed it had successfully tested a hydrogen bomb that could be mounted onto a long-range missile, which analysts say is a major advancement in its nuclear programme.
"The explosive power of the North's nuclear test is estimated to be 50 kilotons," a senior ministry official told lawmakers at an emergency parliamentary briefing.
That would make it five times the size of the North's previous test in September last year and more than three times bigger than the US device that destroyed Hiroshima in 1945.
The official did not confirm whether the tested device was a hydrogen bomb but said "a variety of nuclear material" appeared to have been used.
The North - which in July carried out two intercontinental ballistic missile (ICBM) launches that apparently brought much of the US mainland into range - has rapidly made progress with its weapons programme in defiance of seven sets of UN sanctions.
US, South Korea respond to new nuclear threat
South Korea and the United States will deploy more of the anti-missile defences hated by China in response to Sunday's nuclear test by North Korea, Seoul's defence ministry said on Monday.
Two launchers of the Terminal High-Altitude Area Defense (THAAD) system are already operational, but public concern about the possible environmental impact of the US system forced Seoul to suspend the installation.
"Four remaining launchers will soon be temporarily deployed through consulations between South Korea and the US to counter growing nuclear and missile threats from the North," the ministry said in a statement.
The THAAD launchers are sited on a golf course-turned-US military base in Seongju County, 300 kilometres south of Seoul.
The deployment has infuriated China, which has long argued it will destabilise the region and has retaliated against Seoul through unofficial economic sanctions.
But China said on Monday it had lodged an official protest with North Korea following Pyongyang's largest-ever nuclear weapons test.
China has "launched stern representations with the person in charge of the DPRK embassy in China", foreign ministry spokesman Geng Shuang told reporters at a regular press briefing, using an acronym for the North's official name.
