Its nuclear weapons are being improved "in quality and quantity", an official quoted by the state news agency KCNA said.

The director of North Korea's Atomic Energy Institute said atomic scientists had "made innovations day by day" to "guarantee the reliability of the nuclear deterrent...as required by the prevailing situation".

He added: "In the meantime, the US anachronistic hostile policy toward the DPRK that forced it to have access to the nuclear weapons has remained utterly unchanged and instead it has become all the more undisguised and vicious with the adoption of means openly seeking the downfall of the latter's social system.

"If the US and other hostile forces persistently seek their reckless hostile policy towards the DPRK and behave mischievously, the DP

The words come a day after Pyongyang's warning that it is ready to launch "satellites" on rockets banned by the west. This would mark the ruling communist party's anniversary in October and could put pressure on the US to resume talks.

The North's National Aerospace Development Administration director said the satellite would be used for weather forecasts, adding that space development is "a sovereign state's legitimate right" and that the North Koreans are "fully determined to exercise that right".

He added that the world will "clearly see a series of satellites soaring into the sky at times and locations determined" by the Workers' Party.

There is very little for many residents to return to.

The small mountain community, just north of Napa wine country, took the full force of the Valley Fire that continues to rage across Northern California.

At least one person died here, an elderly woman who could not escape and was beyond the reach of emergency crews.

Her neighbours had raced to safety as the fire - described by officials as the most destructive and unpredictable in California for decades - swept in on Saturday.

Hundreds of buildings were destroyed. Much of what survived now stands twisted by the melting heat.

There is a randomness to how some blocks were wiped out and others left standing. An apartment block at the corner of Main Street and Barnes has completely gone.

The Valley Fire is just one of a dozen burning across California. The Butte Fire, a hundred miles to the east, has claimed dozens of homes. Another blaze, the Rough Fire, is bigger than both.

Mr Turnbull's Liberal Party colleagues voted 54-44 to replace Mr Abbott during a surprise ballot on Monday.

He becomes the nation's fourth leader in a little over two years, but despite division in his party, Mr Turnbull says he will lead a "very strong government".

"There's been a change of prime minister, but we are a very, very strong government, a very strong country with a great potential and we will realise that potential working very hard together," he said.

"This is a turn of events I did not expect, I have to tell you, but it's one that I'm privileged to undertake and one that I'm certainly up to."

Shortly before Mr Turnbull was sworn in, Mr Abbott spoke for the first time since his sudden ousting.

He warned that the persistent volatility in Australia's government could hurt the nation's standing on the global stage.

This is already an intercontinental crisis but it could turn into a European catastrophe if previously open borders are shut.

It will leave tens of thousands, eventually perhaps hundreds of thousands, trapped and homeless within the confines of at least five unwelcoming countries. It will be chaos.

Over the past few days I have travelled with the refugees through the entire Balkan corridor that begins at the Greek border with Macedonia. Travelling through Macedonia and Serbia to Hungary and eventually Austria.

The sheer numbers of people walking, taking buses, trains and taxis is quite remarkable. But things could soon go badly wrong if the EU does not sort out a strategy it is legally obliged to observe.

Like it or loath it the European Union has a collective responsibility to deal with this refugee crisis. The UK and Ireland have opt-outs of course, but the rest of the countries do not, and that is now putting unprecedented stress on the Union.