(Recasts with Xi phone call, world reactions)
* Trump and China's Xi discuss North Korea meeting
* Potentially dramatic breakthrough in nuclear crisis
* Kim committed to denuclearization -South Korean official
By Christine Kim and Jeff Mason
SEOUL/WASHINGTON, March 9 (Reuters) - World leaders welcomed prospects for a possible thaw in the long standoff over North Korea's nuclear weapons program on Friday after U.S. President Donald Trump said he was prepared to hold an unprecedented meeting with North Korean leader Kim Jong Un.
Trump and Kim prompted jitters around the world last year as they exchanged bellicose insults over the North's attempts to develop a nuclear weapon capable of hitting the United States, which it has pursued in defiance of U.N. Security Council resolutions.
But tension eased around last month's Winter Olympics in South Korea, laying the groundwork for what would be the first meeting between leaders from North Korea and the United States, and the biggest foreign policy gamble for Trump since he took office in January last year.
"A meeting is being planned," Trump said on Twitter after accepting an invitation to meet from Kim. There was no date or venue yet for the meeting although it could take place in May.
Chinese President Xi Jinping told Trump in a phone call on Friday that he appreciates his desire to resolve the North Korea issue politically, Chinese state media said.
Xi said he "hopes the United States and North Korea start contacts and dialogue as soon as possible and strive to reach positive results,” the report said.
China is North Korea's largest trading partner and sole major ally, though overall trade has fallen in recent months as U.N. economic sanctions take effect. Trump has frequently tried to enlist Xi's help to rein in Pyongyang.
The head of South Korea's National Security Office, Chung Eui-yong, speaking in Washington on Thursday after briefing Trump about a meeting South Korean officials held with Kim this week, said Trump had agreed to meet the North Korean leader by May in response to Kim's invitation.
Kim had "committed to denuclearization" and to suspending nuclear and missile tests, Chung said.
The talks will take some weeks to arrange, U.S. Secretary of State Rex Tillerson said. A venue was not announced but neutral Switzerland, which often hosts summits, said it was ready to facilitate the meeting.
A meeting between Trump and Kim would be a major turnaround after a year in which North Korea has carried out a battery of missile tests that Washington sees as provocative and after a barrage of insults between the two leaders.
U.S.-based experts say North Korea appeared to show last November that it has succeeded in developing a missile capable of delivering a nuclear weapon anywhere in the United States.
Trump has derided Kim as a "maniac," referred to him as "little rocket man" and threatened in a speech last year to "totally destroy" North Korea, a country of 26 million people, if it attacked the United States or one of its allies.
Kim responded by calling Trump a "mentally deranged U.S. dotard."
Russia, which has joined years of on-again, off-again six-party talks, along with the United States, the two Koreas and Japan, aimed at ending the standoff, welcomed the new, positive signals.
The Japanese government, however, remained cautious.
Japanese Prime Minister Shinzo Abe and Trump, in a call on Thursday, promised to continue to enforce sanctions until Pyongyang took "tangible steps ... toward denuclearization," the White House said in a statement.
German Chancellor Angela Merkel said news of the possible meeting gave reasons to be hopeful about Korea.
"Regarding North and South Korea and also the possibility of a meeting with the president of the United States, you can see that a cohesive international position, including sanctions, can lead to glimmers of hope," she said.
Trump had said previously he was willing to meet Kim under the right circumstances but had indicated the time was not right for such talks. He mocked Tillerson in October for "wasting his time" trying to talk to North Korea.
U.S. Vice President Mike Pence said on Friday that the United States had made "zero concessions" and had "consistently increased the pressure" on North Korea.
A leading Democrat in the U.S. House of Representatives said the Republican president would need help from others in the U.S. government if he is to go head-to-head with Kim over such a complex issue as nuclear weapons and geostrategy.
"It will require the President to rely on the expertise within the State Department, the Intelligence Community, and throughout the government, and not simply on his own estimation of his skills as a ‘deal maker.’" Adam Schiff, the top Democrat on the House Intelligence Committee, said in statement.
South Korean President Moon Jae-In, who led the pursuit of detente with North Korea during his country's hosting of the Winter Olympics, said the summit would set a course for denuclearization, according to a presidential spokesman.
Trump had agreed to meet Kim without any preconditions, another South Korean official said.
"Kim Jong Un talked about denuclearization with the South Korean Representatives, not just a freeze," Trump said on Twitter on Thursday night. "Also, no missile testing by North Korea during this period of time. Great progress being made but sanctions will remain until an agreement is reached."
Trump's aides have been wary of North Korea's diplomatic overtures because of its history of reneging on international commitments and the failure of efforts on disarmament by previous U.S. administrations.
North and South Korea, where the United Sates stations 28,500 troops, are technically still at war after their 1950-53 conflict ended in a ceasefire, not a truce.
Tensions over North Korea rose to their highest in years in 2017 and the Trump administration said all options were on the table, including military ones, in dealing with Pyongyang.
Daniel Russel, until last April the assistant secretary of state for East Asia and the Pacific, the most senior U.S. diplomatic position for Asia, said he wanted to see detail and hear from North Korea on the plans for a meeting.
"Also remember that (North Korea) has for many years proposed that the president of the United States personally engage with North Korea's leaders as an equal - one nuclear power to another," he said. "What is new isn't the proposal, it's the response."
Some U.S. officials and experts worry North Korea could buy time to build up and refine its nuclear arsenal if it drags out talks with Washington.
Asian stock markets rose on the news of the planned meeting, with Japan's Nikkei ending up 0.5 percent and South Korean stocks more than 1 percent higher.
Reporting by Christine Kim in Seoul, and Jeff Mason, David Brunnstrom, Matt Spetalnick, Steve Holland, Yara Bayoumy and John Walcott in Washington Additional reporting by Eric Beech, Mohammad Zargham, Susan Cornwell and Susan Heavey in Washington, Kaori Kaneko in Tokyo and Ben Blanchard in Beijing Writing by Alistair Bell Editing by Paul Tait, Nick Macfie and Frances Kerry