Pompeo in Pyongyang as diplomatic dance speeds up

America's top diplomat Mike Pompeo arrived in Pyongyang today, landing at the centre of a whirlwind of diplomacy ahead of a planned US-North Korea summit.

Pompeo was dispatched on an unannounced visit -- his second in weeks, but first as secretary of state -- to lay the groundwork for Donald Trump's unprecedented meeting with Kim Jong Un.

His visit comes as rumours fly over three US citizens being held in the North, with suggestions they may have been moved in preparation for a release.

The rapid detente on the Korean peninsula triggered by the Winter Olympics is a marked contrast from last year, when Kim and Trump traded personal insults and threats of war over the North's nuclear and ballistic missile programmes.

"We think relationships are building with North Korea," Trump said in a televised address from the White House. "We will see how it all works out. Maybe it won't. But it can be a great thing for North Korea, South Korea and the entire world." But the details of a deal appear to be far from clear.

At a historic meeting inside the Demilitarized Zone last month, Kim and South Korean President Moon Jae-in reaffirmed their commitment to a "common goal" of "complete denuclearisation" of the Korean peninsula.

Hours before Pompeo landed, Trump yanked the US from a nuclear deal with Iran, complicating the prospects of persuading Pyongyang to surrender its atomic arsenal.

On Tuesday, Kim met Chinese President Xi Jinping in China -- the second time in six weeks -- highlighting efforts by the Cold War-era allies to mend frayed ties, and with Beijing keen to avoid being left out in the cold.

China's official Xinhua news agency cited Kim as telling Xi there was no need for North Korea to be a nuclear state "as long as relevant parties abolish their hostile policies and remove security threats against" the country.

Kim also expressed hope that the US and North Korea would take "phased and synchronous measures", signalling Pyongyang wanted a quid pro quo.

Pompeo's itinerary -- including who he would meet in Pyongyang -- was not clear.

He told reporters he would look to prepare for the summit between Trump and "Chairman Un", prompting derision from observers.

"Pompeo doesn't know the surname is Kim, but he's definitely on top of all the conceptual and semantic nuances associated with the phrase 'denuclearisation of the Korean Peninsula'," arms control specialist Jeffrey Lewis tweeted derisively.

Trump's withdrawal of the US from the landmark deal curbing Iran's nuclear programme and reimposing crippling sanctions -- in defiance of European pleas -- also raised concerns.

Trump poured scorn on the "disastrous" 2015 accord, reached after a decade and a half of careful diplomacy by Britain, China, France, Germany, Iran, Russia and past US administrations, describing it as an "embarrassment" to the United States.

"This just makes my head explode," said Robert Kelly of Pusan National University. "Only a fool would trust the US to keep its word in a rogue state nuke deal now."