US President Donald Trump has offered to invite Democratic People's Republic of Korea's top leader Kim Jong-un to visit the White House, if their summit next week goes well, but also threatened to quit the talks and ramp up sanctions pressure if negotiations go otherwise.

At a White House news conference with Japanese Prime Minister Shinzo Abe, Trump said, when asked if he will be inviting Kim to the US if the summit does go well, "The answer is yes … but certainly if it goes well. I think it would be well received. I think he would look at it very favorably, so I think that could happen."

He later said the visit may start with the White House.

Trump said his summit with Kim, scheduled for next Tuesday in Singapore, is "much more than a photo-op". It's rather "a process", from which he predicted "a terrific success or a modified success".

"I think it's not a one-meeting deal," he said. Asked how many days he is willing to stay to talk with Kim, Trump said, "One, two, three — depending on what happens."

The president said he was "totally prepared to walk away" if he felt he must, and would reinstate "maximum pressure" if the talks did not go well.

"All I can say is I am totally prepared to walk away. I did it once before," Trump said. But he added, "And again, I say that with hope that maybe something can happen."

Trump reiterated that he doesn't use the term "maximum pressure" because the two sides are entering into a "friendly negotiation". However, he warned he could impose new sanctions.

"We have many, many sanctions to go, but I don't want to use them unless it's necessary," Trump said. "You'll know how well we do in the negotiation. If you hear me saying, 'We're going to use maximum pressure,' you'll know the negotiation did not do well, frankly."

As to the preparation for the summit, Trump said he didn't think he had to prepare very much. "It's about attitude. It's about willingness to get things done."

Trump also said he promised to raise with Kim the subject of Japanese citizens abducted by DPRK, after Abe focused on the issue during their meeting early in the day.

"He talked about it long and hard and passionately and I will follow his wishes and we will be discussing that with North Korea absolutely, absolutely," Trump said.

Michael J. Green, senior vice-president for Asia and Japan Chair at the Center for Strategic and International Studies, said on Thursday that he believed Trump would bring up the abductee issues, but the Japanese are particularly worried that he will cut a deal where Pyongyang perhaps freezes missiles that have the range to hit the US but not the hundreds of missiles that have the range to hit Japan.

The foreign ministers of DPRK and Singapore on Thursday also discussed the preparations for the upcoming DPRK-US summit, the Xinhua News Agency quoted the KCNA as saying on Friday local time. It is the first time for the DPRK's official media to report on the upcoming Kim-Trump meeting.

US Secretary of State Mike Pompeo said that to prepare for the summit, there have been near-daily briefings in the past few months.

Pompeo said at a news briefing on Thursday that in his previous role as CIA director, "There were few days that I left the Oval Office, after having briefed the president, that we didn't talk about North Korea."

The State Secretary said Trump will reject anything short of "complete, verifiable and irreversible denuclearization of the Korean Peninsula" and the US would work to guarantee North Korea's security should it denuclearize.

Pompeo will accompany Trump to participate in the June 12 summit in Singapore, visit the city state and then travel to South Korea and China, according to a State Department statement.