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.

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 campaign, has been compelled to take a more hardline posture after assuming office in February.

The Korean peninsula has been caught in a cycle of escalating tensions since North Korea'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 more 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.

The jets were reportedly flown out of the US air base in Okinawa, Japan.

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 this year's exercise.

The annual gathering of the North's Supreme People's Assembly usually scores low on important policy announcements -- its role largely limited to unanimously pushing through pre-decided budgets and personnel changes.

But with North Korea having declared itself in a "state of war" with the South, 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.

"It will probably issue some kind of symbolic statement, like urging all North Koreans to stand ready for a possible war," Cho said.

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 that any provocation would swiftly escalate into an all-out nuclear conflict.

Both South Korea and the United States chose to downplay the announcement as just another in a long line of rhetorical provocations.

One threat that grabbed more attention related to the possible closure of a joint-Korean industrial complex which lies inside North Korea.

The Kaesong estate -- established in 2004 as a symbol of cross-border cooperation -- is a crucial source of hard-currency revenue for North Korea which has never allowed past crises on the peninsula to impact its operations.

On Saturday, the North's state body in charge of the complex said it would shut Kaesong down completely if South Korea continues to affront Pyongyang's "dignity".

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 true bellwether of inter-Korean relations, and its closure would mark a significant escalation of tensions beyond all the military rhetoric.