(Rewrites throughout with Trump, senior U.S. officials and Japanese Prime Minister Abe quotes)
* Meeting with Kim Jong Un is being planned -Trump on Twitter
* Potentially dramatic breakthrough in nuclear crisis
* Kim committed to denuclearization -South Korean security adviser
* Japan, U.S. continue 'maximum pressure' on North -PM Abe
WASHINGTON, March 8 (Reuters) - President Donald Trump said on Thursday he was prepared to meet North Korean leader Kim Jong Un for the first U.S.-North Korea summit, marking a potentially dramatic breakthrough in nuclear tensions with Pyongyang.
Kim has committed to "denuclearization" and to suspending nuclear or missile tests, South Korea's National Security Office head Chung Eui-yong told reporters at the White House after briefing Trump on South Korean officials' meeting with Kim on Monday.
"A meeting is being planned," Trump tweeted after speaking to Chung, who announced that Trump expressed a willingness to sit down with Kim in what would be his biggest foreign policy gamble since taking office.
Chung said Trump, in response to Kim's invitation, had agreed to meet by May, and a senior U.S. official later said it could happen "in a matter of a couple of months, with the exact timing and place still to be determined."
Trump has previously said he was willing to meet Kim under the right circumstances but had indicated that the time was not right for such talks. He mocked U.S. Secretary of State Rex Tillerson in October for "wasting his time" trying to talk to North Korea.
Earlier Thursday, Tillerson had said on a visit to Africa that although "talks about talks" might be possible with Pyongyang, denuclearization negotiations were likely a long way off.
"Kim Jong Un talked about denuclearization with the South Korean Representatives, not just a freeze," Trump said in a message on Twitter on Thursday night. "Also, no missile testing by North Korea during this period of time."
Trump added: "Great progress being made but sanctions will remain until an agreement is reached."
A meeting between Kim and Trump, who have exchanged bellicose insults in the past year that have raised fear of war, would be a major turnaround after a year in which North Korea has carried out a battery of tests aimed at developing a nuclear-tipped missile capable of hitting the U.S. mainland.
"Kim pledged that North Korea will refrain from any further nuclear or missile tests," Chung said, apparently referring to a suspension during the duration of any talks.
"He expressed his eagerness to meet President Trump as soon as possible," he said.
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 the administrations of President Bill Clinton, President George W. Bush and President Barack Obama.
Under Clinton in October 2000, then-U.S. Secretary of State Madeleine Albright held talks in Pyongyang with then-North Korean leader Kim Jong Il, the father of Kim Jong Un.
U.S. officials and experts, speaking to Reuters before Thursday's announcement, had cautioned that North Korea could buy time to build up and refine its nuclear arsenal, including a warhead able to survive re-entry into the Earths atmosphere, if it manages to drag out any talks with Washington.
JOINT MILITARY EXERCISES
Chung and National Intelligence Service chief Suh Hoon flew to Washington to explain North Korea's stance on possible future talks with Washington and the prospect of Pyongyang suspending nuclear tests if the security of the North's government is assured.
In what would be a key North Korean concession, Chung said Kim "understands that the routine joint military exercises between the Republic of Korea and the United States must continue."
Pyongyang had previously demanded that such joint drills be suspended in order for any U.S. talks to go forward.
Trump in the past has derided the North Korean leader as a maniac, referred to him as little rocket man and threatened in a speech to the United Nations last year to "totally destroy" his country of 26 million people if it attacked the United States or one of its allies.
Kim had responded by calling the U.S. president a mentally deranged U.S. dotard.
Trump has also been scathing in his criticism of previous U.S. administrations for not doing more to rein in the North Korean government.
"He believes that he has them on the ropes, or at a disadvantage right now. They only made the gesture because they feel the pressure badly and so this a good time," a second senior administration official said.
But U.S. officials may also be wary since North Korea has yet to weigh in directly on its diplomatic offer and previous overtures from Pyongyang have sometimes carried demands that Washington has found impossible to accept, such as the withdrawal of U.S. troops from South Korea.
Japanese Prime Minister Shinzo Abe said he confirmed in telephone talks with Trump that pressure still needed to be applied worldwide on North Korea.
Abe also told reporters he hoped to visit the United States as early as next month to meet Trump to discuss North Korea, among other issues.
"We welcome the change in North Korea's stance", Abe said. "Japan and the United States will not waver in its firm stance that they will continue to put maximum pressure until North Korea takes concrete action towards the complete, verifiable and irreversible end to nuclear missile development."
A senior administration official said Trump agreed to meet Kim because Kim is the "one person who is able to make decisions under their authoritarian, uniquely authoritarian, or totalitarian system," a senior administration official said.
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, noted there had still been no public comment from North Korea about the content of Mondays meeting with the South Korean delegation.
"Lets hear from the North Koreans themselves what they are proposing and what they are willing to do. There is plenty of reason to be cautious, given their track record," Russel said. "Second, lets read the fine print. The North has made peace overtures in the past that did not hold up under scrutiny."
Tensions over North Korea rose to their highest in years in 2017, and the Trump administration has warned that all options are on the table, including military ones, in dealing with Pyongyang, which has pursued its weapons programs in defiance of ever tougher U.N. sanctions.
Signs of a thaw emerged this year, with North and South Korea resuming talks and North Korea attending the Winter Olympics. During the Pyongyang talks this week, the two Koreas agreed to hold their first summit since 2007 in late April.
South Korean President Moon Jae-in has said sanctions should not be eased for the sake of talks and that nothing less than denuclearization of North Korea should be the final goal for talks.
China's foreign minister, Wang Yi, called on the United States and North Korea to hold talks as soon as possible, warning at a news briefing in Beijing on Thursday that things "will not be smooth sailing".
(Reporting Jeff Mason, David Brunnstrom, Matt Spetalnick, Steve Holland, Yara Bayoumy and John Walcott in WASHINGTON; Christine Kim in SEOUL; additional reporting by Eric Beech, Mohammad Zargham, Susan Cornwell in WASHINGTON and Chang-Ran Kim in TOKYO; Editing by Grant McCool)