Shinzo Abe

He said he did not want his illness to get in the way of decision making, and apologised to the Japanese people for failing to complete his term in office.

The 65-year-old has suffered for many years from ulcerative colitis, an inflammatory bowel disease, but he said his condition had worsened recently.

Last year, he became Japan's longest serving prime minister. His current period in office began in 2012.

He will remain in his post until a successor is chosen.

On hearing of his victory he said he would "firmly deal with" threats from North Korea.

The public broadcaster NHK put Mr Abe's Liberal Democratic Party-led (LDP) coalition at 312 seats, allowing it to retain its two-thirds "super majority".

This is vital to his ambition to revise Japan's post-war pacifist constitution.

Article 9 of the constitution, enacted by the country's American occupiers in 1947, calls for the complete renunciation of war.

Japanese forces launched a surprise attack on the base in 1941, killing 2,300 US servicemen and propelling the US into World War Two.

Mr Abe will visit on 27 December with US President Barack Obama.

The announcement, two days before the 75th anniversary of the attack, follows a visit by Mr Obama to Hiroshima.

He became the first US president to visit the Japanese city, where about 150,000 people are believed to have been killed in 1945 by a US atomic bomb.

"I am very honored to see the President elect ahead of other world leaders," Abe told reporters before his departure.

"The Japan-US alliance is the axis of Japan's diplomacy and security. The alliance becomes alive only when there is trust between us. I would like to build such a trust with Mr Trump."

Prime Minister Abe, like other Asian leaders, is keen to find out to what extent Trump's campaign trail rhetoric will become policy after Trump suggested he may withdraw US troops from the region.

Mr Abe described the 90-minute meeting in Trump Tower, New York, as "candid", with a "warm atmosphere".

Some of Mr Trump's campaign rhetoric cast doubt over long-standing US alliances, including with Japan.

The meeting was Mr Trump's first face-to-face with a world leader since winning the presidential election.

The US and Japan have been key allies since the end of World War Two, when the US helped Japan rebuild its economy.

About half the seats in the upper house of parliament were up for grabs.

Prime Minister Shinzo Abe is hoping for backing for his economic policies.

If he can achieve a two-thirds majority in the upper house, to match that in the lower house, he could also hold a referendum on constitutional change, easing constraints on military action.

Polling stations closed at 20:00 local time (11:00 GMT).

President Obama will be traveling there today to offer a reconciliatory balm for the still-painful knowledge of the devastation countries can inflict upon one another.

By visiting Hiroshima, Obama is casting the powerful presidential spotlight on the haunted memories of one of history's darkest days. He also hopes to remind the world that nuclear weapons remain a global threat when placed in the wrong hands.