Scott Morrison is under pressure to provide “straight answers” after agreeing to help Donald Trump with a controversial United States inquiry into the FBI.

The Prime Minister has promised Australia will co-operate with the investigation after speaking with the US president three weeks ago.

“The Australian government has always been ready to assist and co-operate with efforts that help shed further light on the matters under investigation,” a government spokesman said.

“The PM confirmed this readiness once again in conversation with the president.”

The inquiry is widely seen as an attempt to discredit an earlier investigation into Russian interference in the 2016 US election.

Pressure is now on the prime minister to give a full account of the telephone call.

Opposition Leader Anthony Albanese said the Prime Minister needed to give straight answers about his talks with Mr Trump.

“I think Scott Morrison needs to be very clear about the circumstances around this phone call, what was said and whether any agreement was reached in terms of assistance,” he told reporters.

“What Scott Morrison can’t do here is do what he’s had a tendency to do in recent times, which is to dismiss questions which are legitimate from the media as just gossip or as just being in the bubble.

“He needs to actually give some straight answers to what are very clear questions.”

Foreign Minister Marise Payne has defended Australia’s involvement in the investigation.

“I don’t see it as Australia being dragged into a US political issue,” she told ABC radio on Wednesday.

“The inquiry, very much like the others which have been ongoing in the United States, is a matter for them.

“We are conducting ourselves as you would expect us to do in these circumstances, we are working in Australia’s interests, and we are working with our closest and most important ally.

“We should assist them as we can, we ensure that assistance is appropriate, and that is what we are doing.”

Senator Payne said it was up to the US to decide if the full transcript of the telephone call would be released.

Australia’s former high commissioner to the UK Alexander Downer played a significant role in sparking the FBI investigation into Russian interference.

Mr Downer met with George Papadopoulos, a Trump campaign adviser, who told him about damaging Russian information about rival Hillary Clinton.

Greens leader Richard Di Natale said Mr Morrison had sacrificed Mr Downer’s reputation to indulge the US president’s bizarre conspiracy theory.

“It’s clear that Scott Morrison’s thrown Alexander under the bus, there’s no question about that,” he said.

Australia’s ambassador to Washington, Joe Hockey, has also formally offered Australia’s with the investigation.

He said while Mr Downer was no longer employed by the government, Australia stood ready to provide the investigation with all relevant information to the inquiry.

Mr Trump’s call to Mr Morrison comes after revelations the US president called Ukraine’s leader Volodymyr Zelenskiy to ask for help investigating Democrat presidential hopeful Joe Biden.

The US House of Representatives has opened an impeachment inquiry into Mr Trump centring on that call.

-AAP