North Korea preparing for nuclear test, satellite images show

US-based monitoring group 38 North said satellite imaging appears to show the country's nuclear site is "prime and ready" for a new test.

He was referring to a strike group headed by the USS Carl Vinson supercarrier that has been re-routed to the Korean peninsula in a show of force against Kim.

Regarding the U.S. Navy strike force's arrival in the western Pacific, Lu said: "We hope all parties will refrain from irresponsible actions that would be very risky at the moment".

"I was very impressed with President Xi", said Trump.

China's calls for calm come as tensions have risen with the dispatch of a US aircraft carrier to the area and the deployment of thousands of USA and South Korean troops, tanks and other weaponry for their biggest-ever joint military exercises.

Xi seemed to reaffirm his intention to help curb North Korea's nuclear threats to USA forces overseas and allies in the region with a phone call to Trump on Tuesday night in which he signalled a new willingness to act.

Visits by foreign journalists to North Korea are rare and tightly coordinated, and security checks at events attended by leader Kim Jong Un are especially rigorous.

Japan's Nikkei business paper said on Thursday that the United States had sent a "sniffer" plane to Japan to monitor any nuclear tests.

The aircraft, which has been used before to monitor North Korean rocket launches, has arrived on the southern Japanese island of Okinawa, the paper said, citing a Japanese self-defence force official.

A nuclear detonation would be the biggest test yet of Mr Trump's more aggressive stance towards North Korea, coming soon after he warned that the USA was prepared to address North Korea's nuclear threat without help from China, the North's biggest ally and economic partner. If China decides to help, that would be great.

"China insists on realizing the denuclearization of the peninsula, insists on maintaining peace and stability on the peninsula, and advocates resolving the problem through peaceful means", Xi was quoted as saying.

In another tweet, he sought to persuade Xi to put pressure on North Korea in exchange for a good trade deal with the U.S. He wrote: "I explained to the President of China that a trade deal with the U.S. will be far better for them if they solve the North Korean problem!" "Very powerful", Trump said Wednesday.

"If that's the only option the Chinese leave us, there's a real possibility that Chinese entities will get hit", the official said. They said flows of North Korean trucks had slowed to a trickle of three or four a day since February, down from the usual 20. "Many sources indicate Washington is increasing the pressure in the hope that Pyongyang will change course without a shot being fired".

The US show of force, sending what Trump called an "armada" of military vessels towards the region, and North Korea's angry response, has raised fears of a military confrontation.

The worry is that the reclusive North could hold its sixth nuclear test or more missile launches in defiance of United Nations sanctions around the time of its founder's birth anniversary.