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 United Nations Security Council is holding an emergency meeting to discuss its response.
It last imposed sanctions in August, targeting North Korean exports.
Speaking before the meeting, the British ambassador to the UN, Matthew Rycroft, told reporters there were "further sectors of the economy which could be sanctioned" and more could be done to prevent the North Korean government exploiting North Korean labourers working in other countries.
Labels:
