South Korea's new president on Monday promised a strong military response to any North Korean provocation after Pyongyang announced that the two countries were now in a state of war.
President Park Geun-Hye's warning came as North Korea's rubber-stamp parliament was set to hold its annual session and a day after ruling party leaders vowed to enshrine Pyongyang's right to nuclear weapons in law.
It also came as the US announced it had deployed stealth fighters to South Korea as part of an ongoing joint military exercise.
In a meeting with senior military officials and Defence Minister Kim Kwan-Jin, Park said she took the near-daily stream of bellicose threats emanating from the North over the past month "very seriously".
"I believe that we should make a strong and immediate retaliation without any other political considerations if (the North) stages any provocation against our people," she said.
Park, a conservative who had advocated cautious engagement with the North during her election campaign, has been compelled to take a more hardline posture after assuming office in February.
The defence minister made it clear that the South would carry out pre-emptive strikes against the North's nuclear and missile facilities in the event of hostilities breaking out.
"We will... establish a so-called 'active deterrence' aimed at neutralising the North's nuclear and missile threats quickly," Kim said.
The Korean peninsula has been caught in a cycle of escalating tensions since the North's long-range rocket launch in December which its critics condemned as a ballistic missile test.
United Nations sanctions were followed by a nuclear test in February, after which came more sanctions and apocalyptic threats from Pyongyang as South Korea and the United States conducted joint military drills.
Those threats have run the gamut from limited artillery bombardments to pre-emptive nuclear strikes, and have been met with warnings from Seoul and Washington of severe repercussions.
The US military said Monday it had deployed F-22 Raptor stealth fighters to South Korea as part of the ongoing "Foal Eagle" military exercise.
North Korea has already threatened to strike the US mainland and US bases in the Pacific in response to the participation of nuclear-capable US B-52 and B-2 stealth bombers in the exercise.
The US Saturday said it took the "state of war" announcement "seriously" but noted the North's "long history of bellicose rhetoric". China and Russia are among those who have also called for calm.
Australian Foreign Minister Bob Carr said Monday the major concerns were "the belligerence in their statements accompanied by the development of a ballistic missiles capacity and the testing of nuclear weapons".
The annual gathering of the North's Supreme People's Assembly, or parliament, usually scores low on policy announcements -- its role limited to unanimously pushing through pre-decided budgets and personnel changes.
But Monday's session will be closely watched for any sign of the current crisis impacting on the fortunes of members of the ruling elite.
"The North has played most of its political cards, so I don't see any fresh, tangible threats to come out after the meeting," said Cho Han-Bum, an analyst at the Korea Institute for National Unification.
The parliament session was preceded by a gathering Sunday of the central committee of the ruling Workers' Party, chaired by North Korea's young leader Kim Jong-Un.
The meeting declared that the North's possession of nuclear weapons "should be fixed by law", and that its nuclear arsenal should be beefed up "qualitatively and quantitatively".
On Saturday, North Korea announced it had entered a "state of war" with South Korea and warned that any provocation would swiftly escalate into an all-out nuclear conflict.
It also threatened to shut down the joint-Korean Kaesong industrial complex which is a crucial source of hard-currency revenue for Pyongyang and has been shielded from previous crises.
The border crossing to Kaesong, which lies 10 kilometres (six miles) on the North side, was functioning normally on Monday.
The operating stability of the complex is seen as a bellwether of inter-Korean relations, and its closure would mark a clear escalation of tensions beyond all the military rhetoric.