South Korea says it has seen indications that the North is preparing more missile launches, possibly an intercontinental ballistic missile.
It said it was strengthening its controversial US-made Thaad missile defence system after the North’s test of a nuclear bomb at the weekend.
The South has carried out live-fire exercises in response to the test.
The US has warned that any threat to itself or its allies will be met with a “massive military response”.
The North says it tested a hydrogen bomb that can fit on to a long-range missile.
Pyongyang has repeatedly defied UN sanctions and international pressure by developing nuclear weapons and testing missiles, and the provocations have only intensified.
In the past two months it has conducted intercontinental ballistic missile tests, sending one over mainland Japan into the Pacific Ocean. It has also threatened to fire missiles towards the US Pacific territory of Guam.
Chang Kyung-soo, a defence ministry official, told parliament: “We have continued to see signs of possibly more ballistic missile launches. We also forecast North Korea could fire an intercontinental ballistic missile (ICBM).”
The ICBM could be fired into the North Pacific, officials said.
No timeframe was given for any launches but this Saturday, the anniversary of the foundation of the North’s regime, or 10 October, the establishment of the ruling Workers’ Party of Korea, were possible dates.
The ministry also told parliament the US would seek to deploy a nuclear-powered aircraft carrier to seas off the peninsula.
The South’s Defence Minister Song Young-moo said it was now presumed the North had reduced its nuclear warhead in size to below 500kg (1,100lbs), and would be able to fit one on an ICBM.
However, analysts have said the North’s claims about miniaturisation should be treated with considerable caution.
The ministry said there would be more live-fire drills in the South this month, involving Taurus air-to-surface missiles mounted on F-15 jets.
Monday’s drills simulated the targeting of the Punggye-ri nuclear site in Kilju County, where North Korea carried out its bomb test.