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.
The United Nations Security Council is to hold an emergency meeting later on Monday to discuss its response.
Ahead of that meeting, South Korea and Japan's leaders had agreed to push for a stronger UN resolution on North Korea, said a South Korean presidential palace spokesman.
The Security Council last imposed sanctions in August, targeting North Korean exports.
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."
No timeframe was given for any launches.
The ministry also told parliament the US would seek to deploy a nuclear-powered aircraft carrier to seas off the peninsula.
It said it would temporarily deploy four more launchers of the US Thaad (Terminal High-Altitude Area Defence) missile defence system to join the two already at the site in Seongju, south of Seoul.
Both China and Russia are strongly opposed to the Thaad deployment.
The South's defence ministry also said there would be more live-fire drills this month, involving Taurus air-to-surface missiles mounted on F-15 jets.
Monday's drills by the South simulated the targeting of the Punggye-ri nuclear site in Kilju County, where North Korea carried out its bomb test.
"The training demonstrates the South Korean military's resolve to destroy not only the origin of provocation but also the enemy's leadership and supporting forces if they threaten the security of our people," army spokesman Col Roh Jae-cheon is quoted as saying by South Korea's Yonhap news agency.
South Korea and the US had also agreed "in principle" to revise current guidelines so that the South could double the maximum payload of its ballistic missiles, Yonhap also reported.
On Sunday, seismologists started picking up readings of an earth tremor in the area where North Korea had conducted nuclear tests before.
The US Geological Survey put the tremor at 6.3 magnitude.
North Korean state media later confirmed it was no earthquake, claiming it was in fact its sixth and most powerful nuclear test, detonating what it said was a hydrogen bomb that could be loaded on to a long-range missile.