Image copyright Reuters Image caption Mr Pompeo (left) said a "good relationship" had been formed at an April meeting with Mr Kim

US Secretary of State Mike Pompeo is making a surprise visit to North Korea, with speculation growing that he will bring home three jailed Americans.

Mr Pompeo's visit is intended to lay the groundwork for a historic summit between North Korean leader Kim Jong-un and US President Donald Trump.

He told reporters he was hoping to finalise plans for talks, which are likely to focus on denuclearisation.

But the release of the detainees is a central issue for the Americans.

A South Korean presidential official told news agency Yonhap that Pyongyang was expected to release the US citizens as a "goodwill gesture".

Mr Pompeo said he hoped North Korea would "do the right thing" and release three Americans imprisoned in the country.

"We've been asking for the release of these detainees for 17 months," he said.

One of the detainees was jailed in 2015, the other two have been in prison for just over a year. Their convictions have been widely condemned as political and an abuse of human rights.

North Korea has historically used its foreign prisoners as leverage in its diplomatic dealings.

The last American to be freed - Otto Warmbier, who was jailed for stealing a hotel sign - was released last year but was fatally ill, and died shortly after returning home. The cause of death remains unexplained.

The three jailed Americans

Image copyright AFP

Kim Hak-song : Held on suspicion of "hostile acts" in May 2017. He had previously described himself as a Christian missionary who intended to start an experimental farm at the Pyongyang University of Science and Technology (PUST)

: Held on suspicion of "hostile acts" in May 2017. He had previously described himself as a Christian missionary who intended to start an experimental farm at the Pyongyang University of Science and Technology (PUST) Tony Kim, also known as Kim Sang-duk: He also worked at PUST. He was detained in April 2017 on espionage charges. According to South Korean media, he had been involved in humanitarian work in the North.

also known as Kim Sang-duk: He also worked at PUST. He was detained in April 2017 on espionage charges. According to South Korean media, he had been involved in humanitarian work in the North. Kim Dong-chul (pictured above): a pastor in his early 60s, was detained in 2015 on spying charges, and was then sentenced to 10 years hard labour.

Read more: Who are North Korea's American detainees?

North Korea crisis in 300 words

Will we ever know what happened to Otto Warmbier?

This is Mr Pompeo's second trip to Pyongyang since the Trump-Kim meeting was announced.

He said a "good relationship" was formed at the meeting in April, which marked the highest level US contact with North Korea since 2000.

A state department official travelling with Mr Pompeo said the US would also be "listening for signs from North Korea that things have substantially changed" with the nation's nuclear ambitions.

'We have our meeting set'

In April, Mr Trump stunned the international community by accepting Pyongyang's suggestion for direct talks - it will be an unprecedented move for a sitting US president to meet a North Korean leader.

Mr Trump referred to Mr Pompeo's latest visit while announcing that the US was withdrawing from an Obama-era nuclear agreement with Iran.

Media playback is unsupported on your device Media caption Iran nuclear deal: Trump announces US pull-out

"We think relationships are building with North Korea," the president said.

Speaking of his forthcoming meeting with Mr Kim, Mr Trump said: "We have our meeting set. The location is picked. The time and date, everything is picked."

He added: "We will see how it all works out. Maybe it won't. But it can be a great thing for North Korea, South Korea and the entire world."

Amid the diplomatic flurry sparked by the summit, it emerged on Tuesday that Mr Kim visited China this week for talks with President Xi Jinping, the second meeting between the leaders in six weeks.

Image copyright Reuters Image caption Xi Jinping (right) and Kim Jong-un met at the port city of Dalian

They met in the northern city of Dalian and were pictured walking along the seafront together.

Chinese media quoted Mr Kim as saying he hoped "phased and synchronised" measures would be taken to realise denuclearisation on the Korean peninsula.

Meanwhile Chinese Prime Minister Li Keqiang is in Tokyo for talks with South Korea's President Moon Jae-in and Japanese Prime Minister Shinzo Abe, their first trilateral summit in nearly three years.

Mr Abe told his visitors that Japan - which has seen North Korea missiles fly over its territory - was prepared to normalise relations with Pyongyang if the issues of nuclear weapons and of Japanese citizens abducted by the North could be resolved.