North Korea has replaced its long-serving head of state with a close aide to leader Kim Jong Un, promoting a man who was placed on a US sanctions list last year for alleged human rights abuses, state media said Friday.

The move came as part of a top-ranking reshuffle that analysts said further cemented Kim's already unshakeable grip on power.

A Chinese city has opened a new border crossing with North Korea -- fitted with radiation detectors -- even as talks between Washington and Pyongyang have languished over disagreements for nuclear sanctions relief.

In Hanoi, Donald Trump overturned the widely held assumption he would be willing to settle for an interim agreement on North Korea's denuclearization.

The U.S. still believes the "fully verified denuclearization" of North Korea is possible by the end of President Donald Trump's "first term," a senior official said Thursday, despite warnings a key rocket launch site appears to have resumed operations.

The specialized website 38 North and the Center for Strategic and International Studies used commercial satellite imagery to track construction at the site -- which they said began before last week's aborted summit in Hanoi between Trump and North Korea's Kim Jong Un.

Activity has been detected at a North Korean long-range rocket site, suggesting Pyongyang may be pursuing the "rapid rebuilding" of the facility after the collapse of the Hanoi summit, according to analysis of satellite imagery.

Smiling and holding up his clasped hands in a victorious pose, North Korean leader Kim Jong Un on Saturday boarded his private train at the Vietnam-China border for a 60-plus-hour ride home, ending a trip to Vietnam that saw a summit breakdown with President Donald Trump.

He spent his last day in Hanoi laying large red-and-yellow wreaths at a war memorial and at the mausoleum of national hero Ho Chi Minh, surrounded by Vietnamese soldiers in crisp white uniforms and his own entourage of top North Korean officials. At the border, he got out of his armored limousine and clasped his hands, waving to a crowd of people cheering his departure.

The U.S.-North Korea nuclear summit in Hanoi ended abruptly without a deal Thursday, with President Donald Trump saying he had decided to "walk" in the face of Kim Jong Un's demands to drop sanctions.

The much-anticipated second meeting between the two leaders was supposed to build on their historic first summit in Singapore, but they failed to sign a joint statement as initially scheduled and the talks ended in deadlock.

North Korean leader Kim Jong Un's armored train chugged across China on Sunday as he headed to his highly anticipated second summit with U.S. President Donald Trump in Vietnam.

North Korean leader Kim Jong Un expressed "great satisfaction" after receiving a letter from US President Donald Trump ahead of a second summit between the two, Pyongyang's state media reported Thursday.

China's trade with North Korea plummeted last year, data showed Monday, as harsh UN sanctions batter the nuclear-armed country's economy.

Beijing is a key ally of the isolated state and its main source of trade and aid -- but in 2017 it backed United Nations measures to punish Pyongyang over its nuclear and ballistic missile activities.