Anti-North Korean activists protest nuclear test and a rocket launch by Pyongyang, in Seoul, in February 2016
South Korean President Park Geun-Hye on Tuesday signalled a tough new approach to derailing North Korea's nuclear weapons programme, promising an uncompromising and more assertive response to Pyongyang's provocations.
In a strongly-worded televised speech to the National Assembly, Park warned that South Koreans had over the years become "numb" to the threat from their northern neighbour, and said it was time to take a more courageous stand.
North Korea carried out its fourth nuclear test last month and followed that up on February 7 with a long-range rocket launch that was widely condemned as a ballistic missile test banned under UN resolutions.
Arguing that efforts to foster cooperation with the North had led nowhere, Park said it was time to prioritise the stick over the carrot in bringing Pyongyang into line.
"It has become clear that we cannot break North Korea's will to develop nuclear weapons through existing means and goodwill," the president said.
"It's time to find a fundamental solution for bringing practical change in North Korea and to show courage in putting that into action," she added.
Her comments are likely to trigger an angry response from the North, which is already smarting from her unprecedented decision last week to shut down the operations of South Korean firms at the jointly-run Kaesong industrial estate in North Korea.
- Stealth fighter mission -
After her speech Yonhap news agency cited defence officials in Seoul as saying four US F-22 stealth fighter jets would fly a mission over South Korea on Wednesday in a show of force.
Citing the North's nuclear test and rocket launch, Park said it was clear it had no intention of discussing denuclearisation.
"If time passes without any change, the Kim Jong-Un leadership -- which is speeding without a brake -- could deploy a nuclear-tipped missile and we will suffer," she said.
Defending the closure of Kaesong, Park said it was "just the beginning" and signalled further steps that she argued were needed to derail the North's nuclear programme.
"The government will take stronger and more effective measures to make North Korea bitterly realise that it cannot survive with nuclear development and that it will only speed up regime collapse," she said.
A similar line is being pushed by the United States and Japan to try to secure a strong UN Security Council resolution that will include harsh new sanctions on North Korea.
Choi Kang, vice president of the Asan Institute think-tank in Seoul said Park's speech flagged a clear and significant policy change.
"It is a shift from an ideal North Korea policy to a realistic North Korea policy," Choi said.
"In the past, incentive was stressed as the most important means for denuclearising North Korea. Now, by making North Korea pay a practical price, it has shifted to changing North Korea's strategic calculation and inducing it to make a decision," he added.
But Park's speech failed to address the problem the Security Council is having in drafting a resolution that has the support of all five veto-wielding permanent members.
Despite Beijing's annoyance with North Korea's nuclear ambitions and its young maverick leader Kim Jong-Un, its priority has been to prevent chaos on China's border.
It has resisted punitive measures that might push Kim's regime towards collapse.
In her speech, Park indicated that South Korea should be more willing to act unilaterally if other countries lagged behind.
"We must throw away the impotent feeling of relying on the international community's sanctions ... and mobilise all possible methods to solve the problem ourselves," she said.
South Korea is due to begin talks with the US this week on the possible deployment of an advanced US missile defence system which China and Russia have warned could undermine stability in East Asia.
Defence officials in Seoul and Washington say bringing the Terminal High Altitude Area Defence System (THAAD) to the South has become a clear necessity given the progress North Korea is making on its ballistic missile programme.