Former national security advisor John Bolton privately told the US attorney general last year about concerns that President Donald Trump was essentially granting favours to autocrats, The New York Times reported.

It said the revelations, concerning the leaders of China and Turkey, come in an unpublished book manuscript by Bolton.

The same manuscript says Trump told Bolton that he wanted to continue freezing $391m in security aid to Ukraine until officials there helped to investigate his political rivals, the Times previously reported.

READ | Trump says hasn't 'seen' former advisor's bombshell manuscript

Those allegations have roiled Trump's impeachment trial that is ongoing in the US Senate.

According to the Times' latest report, Attorney General Bill Barr responded to Bolton's concerns by pointing to Justice Department investigations of companies in Turkey and China.

Undue influence

Barr said he himself was worried it appeared Trump had undue influence over what would normally be independent inquiries, Bolton's manuscript said.

Bolton wrote that Barr singled out the president's conversations with his Chinese counterpart Xi Jinping about the Chinese telecommunications firm ZTE.

Trump personally intervened in May 2018 to order his Commerce Department to ease penalties against ZTE that had pushed it to the verge of collapse.

US officials had accused the company of violating sanctions against Iran and North Korea.

Barr also cited remarks Trump made to Turkish President Recep Tayyip Erdogan in 2018 about the US investigation of a Turkish bank, the Times report said.

Trump has shown a preference for authoritarian leaders over Washington's traditional Western allies, a situation underscored by his warm welcomes for Erdogan and Egypt's Abdel Fattah al-Sisi at the White House.

Pressure has mounted on Republicans to call Bolton as a witness at Trump's impeachment trial since the Times reported on his revelations about the Ukraine aid.

White House Press Secretary Stephanie Grisham told Fox Business Network on Monday night that Bolton was "clearly... trying to sell a book".

Trump fired the bushy-moustached Bolton last September.

Bolton's strident opposition to Trump's negotiating with Iran, North Korea and the Taliban eventually forced his ouster - though he insists he resigned.