U.S. President Donald Trump acknowledged on Wednesday it was unclear if his planned summit with North Korean leader Kim Jong Un in Singapore next month would go ahead.
North Korea threw the meeting into doubt, saying it might not attend if Washington continues to demand that it unilaterally abandon its nuclear weapons.
Previous attempts to persuade North Korea to back off its nuclear weapons program have been doomed, in part by the North's concerns about being attacked and enmity between Pyongyang and Washington.
The following describes how Trump and Kim came to the table and how previous talks failed:
2017-2018 TRUMP RHETORIC, HISTORIC PLAN
After becoming president in January 2017, Donald Trump seeks help from Chinese President Xi Jinping in dealing with North Korea. Moon Jae-in, who favours engagement with the North, is elected president of South Korean in May.
As Pyongyang tests missiles capable of reaching the United States, Trump turns up the tension and in a September 2017 U.N. address threatens to "totally destroy North Korea" while mocking Kim as "little rocket man." Kim responds with a threat to "tame the mentally deranged U.S. dotard with fire."
The United States pushes for increased U.N. economic sanctions against North Korea including bans on exports of coal, iron and seafood. Trump addresses the South Korean National Assembly two months later, saying the North will have to take steps toward denuclearization if it wants to talk. He also labels North Korea a state sponsor of terrorism, a designation that had been removed in 2008.
The 2018 Winter Olympics, hosted by South Korea, provide a breakthrough with the North sending a delegation to the games.
In March, Trump and Kim agree to meet. North Korea says it will suspend its missile and weapons testing and, despite past insults, Trump praises Kim as "very honourable."
Trump announces on Twitter on May 10 that the meeting will take place in Singapore on June 12.
North Korea lays out details of a plan to dismantle its Punggye-ri nuclear test site later in May.
2010-2016 DEMISE OF TALKS
North Korea's sinking of a South Korean patrol ship near the nations' maritime border in March 2010 becomes a roadblock to talks. The situation worsens in November when the North fires artillery at a South Korean island, killing two soldiers. The South, United States and Japan reject China's call to resume six-party talks until relations improve.
In 2011, China, Russia and the United States make separate and unsuccessful moves to restart negotiations. North Korean leader Kim Jong Il dies in December and son Kim Jong Un takes power.
Concerns rise in 2013 as the North makes rocketry advances and continues nuclear testing but no more talks are held. The administration of President Barack Obama increases sanctions on Pyongyang.
2003-2009 SIX-PARTY TALKS
In 2003, Kim Jong Il announces Pyongyang will withdraw from the Non-Proliferation Treaty it had agreed to in 1985. Three months later North Korea announces it has a nuclear weapon.
Talks begin in August 2003 between North Korea, South Korea, China, the United States, Russia and Japan.
In 2004-05, as the talks are held intermittently, North Korea continues missile testing. As would become a pattern, Pyongyang offers to curtail its work in exchange for aid.
With the talks in abeyance until 2006, the North accuses the United States of being a nuclear menace, drawing a warning from President George W. Bush.
In 2007, North Korea promises to shut its nuclear reactor in exchange for fuel oil. It later demands the United States release US$25 million in frozen funds, which it gets, clearing the way for more talks.
A North Korean pledge to disclose all its nuclear activities by the end of 2007 goes unfulfilled.
In May 2008, North Korea demands the United States remove it from a list of state sponsors of terrorism. Washington complies in October, prompting the North to resume dismantling its Yongbyon nuclear plant.
In 2009, the U.N. Security Council responds to a missile test by threatening to increase sanctions and Pyongyang says it will no longer participate in six-party talks.
1994-2002 NORTH KOREA-U.S. TALKS UNDER CLINTON, BUSH
In 1994, North Korea and the United States, under President Bill Clinton, sign an "agreed framework" with the goal of freezing and eventually discontinuing Pyongyang's nuclear program. In exchange, North Korea has the possibility of normalized relations, fuel oil and help building light-water nuclear reactors.
North Korea's production and sale of missiles become an issue. Talks begin with the United States pushing the North to curtail the missile business, while Pyongyang demands financial compensation for lost income. In 1998 sanctions are imposed on the North for sending missile technology and parts to Pakistan.
When Bush becomes president in 2001, Pyongyang detects a more hostile attitude and U.S. sanctions are imposed on a North Korean company for missile-related transfers to Iran.
Relations are frayed further in 2002 when Bush labels North Korea as part of an "axis of evil" sponsoring terrorism and seeking nuclear weapons.
The agreed framework breaks down in December 2002 as the United States determines North Korea has been secretly pursuing nuclear weapons and Pyongyang says it has a right to them for defensive purposes. The North orders international inspectors out of the country while reopening its shuttered nuclear facilities.
(Compiled by Bill Trott; Editing by Grant McCool and Alistair Bell)