As newly-appointed British Prime Minister Liz Truss closed off her first day in office Wednesday just one day after speaking with President Joe Biden via a congratulatory call, the state of the UK's and America's relationship remains uncertain given the two leaders' opposing platforms on key issues. 

Biden and Truss spoke on the phone Tuesday, with the president congratulating Truss on her win and the two countries' "special relationship." A handout from the White House stated the two leaders emphasized their wanting to deepen that relationship while also cooperating on "global challenges," including Russia's war on Ukraine, challenges from China, and preventing Iran from securing a nuclear deal. 

"There’ll be issues that we have to work through, of course, as they’re always are," US National Security Advisor Jake Sullivan said on "The David Rubenstein Show: Peer-to-Peer Conversations."

The leaders did touch upon the Northern Ireland Protocol, a hot-button topic given Truss and Biden's conflicting views on the matter. UK leaders are currently calling for a renegotiation of the original proposal, which leaves Northern Ireland in the European Union's single market along with the Republic of Ireland.  

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Truss previously proposed enacting legislation that could override the original plan, a move certain Unionists support given their stance that the current proposal weakens Northern Ireland's standing.

Liz Truss speaking wearing a dark suit in front of a blue/gray background

Liz Truss meets supporters at a Conservative Party leadership election hustings at the NEC, Birmingham, England, Tuesday, Aug. 23, 2022. After weeks of waiting, Britain will finally learn who will be its new prime minister.  The governing Conservative Party will announce Monday, Sept. 5, 2022, whether Foreign Secretary Liz Truss or former Treasury chief Rishi Sunak won the most votes from party members to succeed Boris Johnson as party leader and British prime minister. (AP Photo/Rui Vieira, File)

However, Biden, along with others in Washington, says the move could risk decades of peace between Northern Ireland and Ireland. Details as to what the two leaders touched upon regarding the Northern Ireland Protocol were not specified in the White House handout but international experts, as well as sources close to Truss, have expressed frustration with Biden's stance on the matter.

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"The White House has been very unhelpful over Northern Ireland, and Nancy Pelosi, speaker of the House, has made several threats to Great Britain in terms of sinking a U.S-U.K trade deal because of the Northern Ireland issue," Nile Gardiner, a former adviser to former British Prime Minister Margaret Thatcher, told Fox News Digital earlier this week.

President Biden speaks in front of two microphones in a suit

President Biden speaks about inflation and supply chain issues in Los Angeles.  (AP Photo/Damian Dovarganes)

An official "familiar with [Truss'] thinking" told Bloomberg Truss has been irritated by Biden's stance on the issue, saying the US should stay out of it given the domestic nature of the negotiations. 

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Biden and Truss, however, did establish they would work together to fight against Russia as well as economic issues resulting from the war on Ukraine.

Split photo of Liz Truss on the left and Joe Biden on the right

Newly appointed British Prime Minister Liz Truss and President Joe Biden spoke on the phone Tuesday to reinforce the "special relationship" between the UK and the US. (Carl Court/Getty Images // Kevin Dietsch/Getty Images)

"I look forward to deepening the special relationship between our countries and working in close cooperation on global challenges, including continued support for Ukraine as it defends itself against Russian aggression," Biden tweeted.

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A meeting between Truss and Biden could be on the horizon, given Boris Johnson's trip to Washington DC almost a year ago. No details have yet to be released, but an in-person meeting is expected closer to the UN's September General Meeting in New York.