Image caption The ruling Workers' Party raised nuclear weapons at its Sunday meeting

North Korea's parliament has endorsed plans to give nuclear weapons greater prominence in the country's defences.

The move came a day after the ruling Workers' Party called for nuclear forces to be "expanded and beefed up qualitatively and quantitatively".

North Korea has said it is entering a "state of war" with the South - prompting Seoul to promise a "strong response" to aggression by the North.

The North is angry at UN sanctions following its nuclear test in February.

It is also unhappy with joint US-South Korea annual military drills. But the US says there is no sign of North Korean "actions to back up the rhetoric".

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

'The nation's life'

North Korea's Supreme People's Assembly convened on Monday for a day-long annual session. It normally focuses on making economic decisions.

But state news agency KCNA said the body had "unanimously adopted an ordinance that provides for giving nuclear weapons greater prominence in the defence of the country".

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 11 Mar : US-South Korea annual joint military drills begin

: US-South Korea annual joint military drills begin 19 Mar: US flies B-52 nuclear-capable bombers over Korean peninsula, following several North Korean threats

US flies B-52 nuclear-capable bombers over Korean peninsula, following several North Korean threats 27 Mar: North Korea cuts military hotline with South

North Korea cuts military hotline with South 28 Mar: US flies B-2 stealth bombers over Korean peninsula

US flies B-2 stealth bombers over Korean peninsula 30 Mar: North Korea says it is entering a "state of war" with South Korea How potent are North Korea's threats?

The law reads that the country's nuclear weapons are a "means of defence" and serve the purpose of "dealing deadly retaliatory blows at the strongholds of aggression until the world is denuclearised".

On Sunday the Workers' Party Central Committee held a rare high-level meeting in which it described nuclear weapons as "the nation's life".

"The DPRK [North Korea]'s possession of nuclear weapons should be fixed by law and the nuclear armed forces should be expanded and beefed up qualitatively and quantitatively," a KCNA report on the meeting said.

"The People's Army should perfect the war method and operation in the direction of raising the pivotal role of the nuclear armed forces in all aspects concerning war deterrence and war strategy."

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

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

But despite its rhetoric 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.

In Washington, President Barack Obama's spokesman that "despite the harsh rhetoric we are hearing from Pyongyang, we are not seeing changes to the North Korean military posture, such as large-scale mobilisations and positioning of forces".