Kim Jong-un has said North Korea's nuclear weapons should be ready for use "at any time", state media report.

He told military leaders North Korea would revise its military posture to be ready to launch pre-emptive strikes, the Korean Central News Agency said.

On Wednesday the UN imposed some of its toughest ever sanctions on North Korea, following its recent nuclear test and missile launch.

In response, the North fired six short-range projectiles into the sea.

KCNA said Kim was speaking at a military exercise on Thursday, which is thought to be when the projectiles were fired.

He said North Korea "must always be ready to fire our nuclear warheads at any time" because enemies were threatening the North's survival.

"At an extreme time when the Americans... are urging war and disaster on other countries and people, the only way to defend our sovereignty and right to live is to bolster our nuclear capability," he was quoted as saying.

Such rhetoric is not unusual from North Korea, but despite its recent nuclear and ballistic tests, analysts still doubt it has the ability to make a nuclear bomb small enough to put on a feasible missile.

Kim Jong-un's announcement brought a swift response from the US.

"We urge North Korea to refrain from provocative actions that aggravate tensions and instead focus on fulfilling its international obligations and commitments," Pentagon spokesman Commander Bill Urban said.

The US and South Korea began talks on Friday on the possible deployment of a US missile defence shield in the South.

Initial talks will focus on the costs, effectiveness and environmental impact of installing the Terminal High Altitude Area Defense (THAAD) system, among other issues, the Yonhap news agency reported.

What exactly is banned?

--The export of coal, iron and iron ore used for North Korea's nuclear or ballistic missile programmes.

--All gold, titanium ore, vanadium ore, rare earth minerals and aviation fuel exports.

--Any item (except food and medicine) that could develop North Korea's armed forces.

--Small arms and light weapons are now included in an arms embargo.

--Upmarket watches, watercraft, snowmobiles and other recreational sports equipment added to a ban on luxury goods.

--No vessels or planes can be leased or registered to North Korea.

What are the other measures?

--Member states must inspect all cargo to and from North Korea, not just those suspected of containing prohibited items.

--An asset freeze on North Korean funds linked to nuclear and missile programmes.

--Foreign financial institutions cannot open new offices in North Korea without approval, and North Korean banks cannot open offices abroad.