US President Donald Trump sought the help of Chinese President Xi Jinping to win re-election, his former national security adviser John Bolton claims in a new book.

At a closed-door meeting, Mr Trump turned to Xi Jinping and “stressed the importance of farmers and increased Chinese purchases of soybeans and wheat in the electoral outcome,” according to excerpts he published in the Wall Street Journal.

Mr Bolton described this as the “confluence in Trump’s mind of his own political interests and US national interests.”

Separately, the New York Times reported that Mr Bolton writes that there were other improper actions by Trump involving Turkey’s Halkbank – an open case in US courts – and the Chinese tech company ZTE.

He accused Mr Trump of treating “obstruction of justice as a way of life,” while blasting lawmakers for “impeachment malpractice” for only focusing on the Ukraine affair in their investigations.

He also alleged Mr Trump had expressed a willingness to halt criminal investigations to give “personal favours to dictators he liked”.

Mr Bolton, Mr Trump’s former national security adviser, refused to testify during the House impeachment hearings, making his claim controversial.

His book is being attacked by Trump loyalists, while liberals see Mr Bolton as someone who could have aided the impeachment inquiry, but held out for a book deal instead.

“Bolton’s staff were asked to testify before the House to Trump’s abuses, and did. They had a lot to lose and showed real courage,” said Congressman Adam Schiff, a leading Democratic investigator during impeachment.

“When Bolton was asked, he refused, and said he’d sue if subpoenaed. Instead, he saved it for a book,” Mr Schiff said. “He’s no patriot.”

Mr Bolton also alleges Mr Trump encouraged Mr Xi to build internment camps for Muslim minority groups.

“Trump said that Xi should go ahead with building the camps, which Trump thought was exactly the right thing to do,” Mr Bolton writes in the book, dubbing them “concentration camps.”

A US federal prosecutor has sued to stop the upcoming publication, slated for next week, saying the work is a breach of Mr Bolton’s confidentiality agreements and a national security threat.

The lawsuit says Mr Bolton stands to earn $US2 million ($A2.89 million).

The book also reportedly seeks to show that Mr Trump is ignorant on key matters of world affairs, such as wondering if Finland is part of Russia and if Britain had nuclear weapons.

The volume, if published, would be the latest in a long line of people who were once close to Mr Trump who have published books in the last four years bashing the President, but Mr Bolton is likely the one who had the most involvement in the inner workings of foreign policy.

-with agencies