Prime Minister Malcolm Turnbull repeatedly declined to publicly support his embattled deputy Barnaby Joyce as he talked up Australia’s mateship with the US during his first day in Washington.

Mr Turnbull, speaking to reporters outside the front gates of the White House on Friday morning (AEDT), could not avoid questions about Mr Joyce despite being half a world away from Canberra.

The Prime Minister initially asked reporters to keep their questions about Mr Joyce to the end of the press conference.

When his staff said there was only one question left, Mr Turnbull declined to say if he still supported the Nationals leader who is now facing historical allegations of sexual harassment and a potential leadership spill.

Mr Joyce is still weathering the fallout from revelations earlier this month that he had an extra-marital affair with a former staffer who is now pregnant with his child.

“These are matters for the leadership of the National Party,” Mr Turnbull eventually answered.

“I’ve been a great pain to stress that I have not, nor has my party, sought to influence in any way the deliberations of the National party.”

Mr Turnbull landed in Washington on Thursday to lead what is the largest and most significant Australian delegation, including state premiers and business leaders, to visit the US.

He met with Treasury Secretary Stephen Mnuchin and Homeland Security Secretary Kirstjen Nielsen on Friday morning, and after speaking to reporters, was scheduled to meet with new Federal Reserve Chairman Jerome Powell and Joint Chiefs Chairman General Joseph Dunford.

Mr Turnbull and wife Lucy will spend most of Friday at the White House with US President Donald Trump and first lady Melania.

Ahead of his meeting with the President, Mr Turnbull penned an opinion piece for American national newspaper USA Today talking up the two nations’ strong bonds.

“Mates stick by each other through good times and bad,” he wrote.

“Mates have each other’s backs.”

The Australian Embassy in Washington has dubbed Mr Turnbull’s trip “100 Years of Mateship” to mark the 100th anniversary of the World War I Battle of Hamel in northern France, where US and Australian troops fought alongside each other for the first time.

“For our two countries, it was the start of an unbreakable alliance that would see Australia fighting alongside the US in every significant conflict in the hundred years that followed,” Mr Turnbull wrote.

A fierce debate over gun control has taken over the US after last week’s Florida school massacre and Mr Trump has signalled he will look at introducing stricter measures.

Mr Turnbull will likely face questions from the White House press corps and possibly Mr Trump on Friday about Australia’s success in responding to the Port Arthur massacre more than 20 years ago.

Mr Turnbull said he would be reluctant to wade into the US gun debate.

“We’ll leave the domestic political debate to the Americans,” Mr Turnbull said.