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.
What has the South said?
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.
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.