Stephen Harper was spotted leaving the West Wing on Monday, but little is known about what the former prime minister said inside the White House.

Harper was expected to meet with Larry Kudlow — the director of the National Economic Council and U.S. President Donald Trump's go-to economic adviser — and John Bolton, the president's national security adviser, according to multiple sources who spoke to CBC News on the condition of anonymity.

An Associated Press photographer snapped a photo of the former Conservative leader leaving the White House, but neither Harper's team nor the U.S. administration has responded to CBC's requests for comment about what happened during Monday's trip to Washington.

Harper did Tweet Monday that he looked forward to meeting with more business and government leaders "to discuss the forces shaping our future," in a plug for his forthcoming book.

Engaging discussions in 5 European capitals last week focused on political disruption & global trends. I look fwd to continuing to meet w/ business & govt leaders to discuss the forces shaping our future in advance of the release of <a href="https://t.co/E9IBBUAiIk">https://t.co/E9IBBUAiIk</a> in a few short months! —@stephenharper

Harper, who now works as a consultant, personally approached the two officials for a meeting, sources told CBC News last week.

The sources said Bolton phoned the Canadian embassy in Washington to make some arrangements ahead of the meeting — a call that caught Canadian officials off guard as they were not expecting such a visit.

CTV News first reported Harper's planned meeting with Bolton in Washington, citing details contained in emails the broadcaster had obtained.

Harper has not shielded away from talking publicly about the Canada-U.S. relationship since leaving office. He has waded into the ongoing NAFTA talks, offering commentary on TV and at conferences.

While he has criticized some of the Liberal government's actions on the trade file, he has defended the merits of NAFTA.

"I can understand why President Trump, why the American people feel they need some better trade relationships," Harper told Fox News last month. "[But] this is the wrong target."

'United' on NAFTA

Foreign Affairs Minister Chrystia Freeland said she believes Harper's appearance on Fox News was helpful.

"When it comes to prime minister Harper, he is someone we all need to respect as the former prime minister of Canada," she said on Friday.

"One of the things which has been incredibly valuable for Canada throughout the NAFTA negotiations and more recently as the [national security] measures have come into effect has been the incredibly united position that Canadians have demonstrated."

Harper also often meets with international conservative leaders as chair of the International Democrat Union, an alliance of conservative and centre-right parties founded 35 years ago by Britain's Margaret Thatcher, Germany's Helmut Kohl and then-U.S. vice president George Bush Sr., among others.

He's also still active as one of the founders of the Friends of Israel Initiative.

Harper's meeting came as the U.S. and Canada battle it out over trade.

Kudlow, who recently suffered a heart attack but is back on the job, even took to American airwaves after the G7 summit in Charlevoix to attack Trudeau's plan to impose retaliatory tariffs on the U.S.

Last month Trump slapped tariffs on Canadian aluminum and steel imports and sought to justify them on national security grounds.

White House Press Secretary Sarah Sanders says "escalating tariffs against the U.S does nothing to help Canada." 0:35

In response, Canada has imposed $16.6 billion worth of new tariffs on a host of U.S. goods, from whisky to ketchup.

"We've been very nice to Canada for many years and they've taken advantage of that, particularly advantage of our farmers," White House Press Secretary Sarah Sanders said on Monday.

"Escalating tariffs against the U.S does nothing to help Canada and it only hurts American workers. The president is working to fix the broken system and he's going to continue pushing for that."

The timeline for renegotiating NAFTA has been kicked further down the line, with Trump suggesting he won't sign a deal until after his country's midterm elections.