US military bases remain essential to Japan's security, Prime Minister Shinzo Abe said in an interview Wednesday, as he brushed off comments by Republican presidential candidate Donald Trump that Tokyo should be left to defend itself.

Japan's alliance with Washington has been the bedrock of its defence since the end of World War II, and the country still hosts 47,000 US troops.

"I cannot conceive of any situation within the foreseeable future when the US presence wouldn't be necessary," Abe told The Wall Street Journal.

Abe has vowed to boost Japan's military but he sidestepped a question on whether Japan would play a bigger role in its own defence, saying Tokyo would strengthen its relationship with Washington.

"By strengthening the Japan-US alliance, we'll strengthen deterrence and that will contribute to peace and stability in the region, not just Japan," Abe said in the interview with the WSJ, conducted in Tokyo on Tuesday.

Abe also said he wanted to push through a huge trans-Pacific trade deal that has been attacked by both Democratic presidential candidates, Bernie Sanders and Hillary Clinton.

"This is the birth of an economic zone that has 40 percent of the world's [gross domestic product], one that is protected by free and fair rules,' Abe told the WSJ, referring to the Trans-Pacific Partnership, a massive multi-nation deal of which Japan and the US are the key players.

"Through it, the US, Japan and the other countries participating in TPP will achieve great profit and gain chances for growth."

Abe's comments come after Trump said that US alliances with countries such as Japan and South Korea cost too much to maintain and that they should be responsible for their own defence -- unless they bear more of the cost burden.

Trump has also suggested that they could develop their own nuclear weapons, a stance particularly controversial in Japan, which is the only country in the world to be attacked with atomic bombs.

Asked at a press conference last week about comments by Trump, Abe said that the alliance with the US remains strong and will not change after the US presidential election in November, comments he reiterated in the interview.

"No matter who will be the next president, the Japan-US alliance is the cornerstone of Japan's diplomacy," he said.

Japan, which is constitutionally barred from waging offensive war, last year passed new laws that could, under certain circumstances, see its troops fight abroad for the first time since the end of World War II.

Abe says the legislation is necessary because of perceived threats from an increasingly assertive China and unstable North Korea.

Opponents argue they go against both the constitution and the national psyche, and could see Japan dragged into far-flung wars led by treaty ally the US.

Washington has backed the changes, but regional rivals China and South Korea have expressed concern at any expansion of Japan's military.