The South and the United States will deploy more of the Terminal High-Altitude Area Defense (THAAD) missile launchers that have infuriated Beijing, the Defense Ministry said, AFP reported.
The announcement came after Seoul fired an early-morning volley of ballistic missiles in an exercise simulating an attack on the North's nuclear test site.
Pictures showed South Korean short-range Hyunmoo missiles roaring into the sky in the pale light of dawn from a launch site on the east coast.
Pyongyang said the device it detonated Sunday was a hydrogen bomb – far more powerful than the fission-based devices it is believed to have previously tested – and small enough to fit into a missile.
South Korean Defense Ministry officials estimated its strength at 50 kilotons – five times the size of the North's previous nuclear test.
Defense Minister Song Young-moo said Seoul believed Pyongyang had succeeded in miniaturizing its nuclear weapons to fit into an ICBM.
Signs that North Korea was "preparing for another ballistic missile launch have consistently been detected since Sunday's test", the ministry said.
It did not indicate when a launch might take place, but said it could involve an ICBM being fired into the Pacific Ocean to raise pressure on Washington further.
After Sunday's test the United States warned it could launch a "massive military response" to threats from North Korea that would be "both effective and overwhelming."
"We are not looking to the total annihilation of a country, namely North Korea," Defense Secretary Jim Mattis said, but warned: "We have many options to do so."
South Korea's President Moon Jae-in called for new United Nations sanctions to "completely isolate North Korea."
At their summit in China, the North's key ally, the five-nation BRICS grouping – taking in the host nation as well as Brazil, Russia, India, and South Africa – said Monday it "strongly deplores" the test.
Moon and Japan’s Premier Shinzo Abe agreed to work for stronger sanctions against the North, but seven sets of UN measures have so far done nothing to deter Pyongyang.
US Treasury Secretary Steven Mnuchin said Sunday his department was preparing measures to "cut off North Korea economically" and ensure anyone trading with it could not do business with the US.
The North hailed the test as "a perfect success."
Hours before the test, the North released images of leader Kim Jong-un inspecting a device it called a "thermonuclear weapon with super explosive power" entirely made "by our own efforts and technology."
The North says it needs nuclear weapons to defend itself against the threat of invasion, and analysts say it is seeking to strengthen its hand for any future negotiations with Washington.
An increasing number of experts say talks are now the only way to address the issue.