Media playback is unsupported on your device Media caption Lucy Williamson reports from the heavily-guarded inter-Korean border

South Korea has promised a "strong response" to North Korean aggression, amid high tensions on the peninsula.

Speaking to defence officials on Monday, President Park Geun-hye said that she took the series of threats from Pyongyang "very seriously".

North Korea said on Saturday that it was entering a "state of war" with South Korea.

On Sunday, the US sent stealth fighters to South Korea, as North Korea pledged to build up its nuclear arsenal.

Meanwhile, North Korea has announced it has appointed a new premier, Pak Pong-ju. He was sacked from the same post in 2007.

Military drills

In her comments, Ms Park said: "If there is any provocation against South Korea and its people, there should be a strong response in initial combat without any political considerations,"

In recent days North Korea has issued multiple warnings of attacks on US and South Korean targets - to which the US has responded with an apparent show of military hardware.

The communist state has been angered both by UN sanctions imposed after its nuclear test in February and the joint US-South Korea annual military drills.

South Korean Foreign Minister Yun Byung-se is expected to meet US Secretary of State John Kerry this week in Washington to discuss North Korea, Yonhap news agency reports.

Timeline: Korean tensions 12 Dec: North Korea fires three-stage rocket, in move condemned by UN as banned test of long-range missile technology

North Korea fires three-stage rocket, in move condemned by UN as banned test of long-range missile technology 12 Feb: North Korea conducts an underground nuclear test, its third after tests in 2006 and 2009

North Korea conducts an underground nuclear test, its third after tests in 2006 and 2009 7 Mar: UN approves fresh sanctions on Pyongyang; North Korea says it has the right to a "pre-emptive nuclear strike" on the US

UN approves fresh sanctions on Pyongyang; North Korea says it has the right to a "pre-emptive nuclear strike" on the US 11 Mar : US-South Korea annual joint military drills begin; North Korea says it has scrapped the Korean War armistice (the UN says the pact cannot be unilaterally scrapped)

: US-South Korea annual joint military drills begin; North Korea says it has scrapped the Korean War armistice (the UN says the pact cannot be unilaterally scrapped) 19 Mar: US flies B-2 nuclear-capable bombers over Korean peninsula, following several North Korean threats to attack US and South Korean targets

US flies B-2 nuclear-capable bombers over Korean peninsula, following several North Korean threats to attack US and South Korean targets 20 Mar: Broadcasters and banks in South Korea hit by cyber attack, the origin of which remains unknown, days after North Korea says some of its sites were hacked

Broadcasters and banks in South Korea hit by cyber attack, the origin of which remains unknown, days after North Korea says some of its sites were hacked 27 Mar: North Korea cuts military hotline with South, the last official direct link between the two

North Korea cuts military hotline with South, the last official direct link between the two 28 Mar: US flies stealth bombers over Korean peninsula; showcasing ability for precision strike "at will"

US flies stealth bombers over Korean peninsula; showcasing ability for precision strike "at will" 30 Mar: North Korea says it is entering a "state of war" with South Korea How potent are North Korea's threats?

The US flew F-22 planes from Japan to South Korea's Osan Air base on Sunday, as part of ongoing joint military exercises with South Korea, officials said.

"[North Korea] will achieve nothing by threats or provocations, which will only further isolate North Korea and undermine international efforts to ensure peace and stability in Northeast Asia," the US military command in South Korea said in a statement reported by Reuters news agency.

In March, the US deployed both B-2 and B-52 planes, which have nuclear capabilities, over South Korea. It said this demonstrated its "capability... to provide extended deterrence to [its] allies in the Asia-Pacific region".

It is not the first time F-22s have been used drills with South Korea, but the move came as North Korea's Central Committee held a rare high-level meeting on Sunday.

The committee described nuclear weapons as "the nation's life" and vowed to further develop its nuclear programme, state-run news agency KCNA said.

"Only when the nuclear shield for self-defence is held fast, will it be possible to shatter the US imperialists' ambition for annexing the Korean Peninsula by force," the report added.

North Korea's Supreme People's Assembly - the rubber-stamp parliament - convened on Monday for a day-long annual session.

While the group normally focuses on making economic decisions, this meeting will be keenly watched given the current high tension.

Few think the North - which last week cut a military hotline which was the last official direct link with Seoul - would risk full-blown conflict.

But in recent years there have been deadly incidents such as the sinking of a South Korea warship (in which Pyongyang denies any role) and the shelling of a South Korean island.

However the jointly-run Kaesong industrial park, which is located within North Korea's borders, remains in operation.

Workers from South Korea were crossing into the park - which is a key money-maker for North Korea - as normal on Monday, reports said.

The complex is seen as a barometer of North-South tensions, observers say, and a move to close it would be seen as an escalation of current tensions.

On Monday, KCNA reported that Mr Pak had been elected to the role of premier of the North Korean cabinet.

He was dismissed in 2007, reportedly after proposing the country adopt a US-style wages system.

The premiership holds little actual power, but analysts say his appointment indicates that he will again be taking a central role in economic decision-making.