The White House has warned again that dismantling the Northern Ireland Protocol would "not create a conducive environment" for US-UK trade talks.
President Joe Biden's press secretary, Karine Jean-Pierre, said such actions could harm the Good Friday Agreement.
The warning comes as the new Northern Ireland secretary said he wanted to "negotiate a solution" with the EU.
However, Chris Heaton-Harris told MPs the government would still legislate if this did not happen.
Speaking to reporters on Wednesday, Ms Jean-Pierre said: "There is a no formal linkage on trade talks between the US and the UK and the Northern Ireland Protocol, as we have said, but efforts to undo the Northern Ireland Protocol would not create a conducive environment."
Mr Biden has previously said that he felt "very strongly" about the protocol and has repeatedly urged the UK not to take any action that could potentially create a hard Irish border.
Mr Biden raised the issue of the protocol directly with Liz Truss on Tuesday during their first phone call.
A read-out of their conversation from the White House said the pair discussed "the importance of reaching a negotiated agreement with the European Union".
The protocol is the part of the Brexit deal that keeps Northern Ireland in the EU's single market for goods, avoiding a hard border with the Republic of Ireland.
The arrangement ensured free trade could continue across the Irish land border, which is a sensitive issue because of the history of conflict in Northern Ireland.
But the protocol brought in some new checks on goods moving between Great Britain and Northern Ireland and has been criticised by unionist politicians.
However, the majority of politicians elected to the Northern Ireland Assembly in May's elections support the arrangements.
The Democratic Unionist Party (DUP) has refused to re-enter a power-sharing government in Northern Ireland until its concerns about the protocol are resolved.
Speaking on her first full day in office, Ms Truss told the Commons that the Northern Ireland Executive must return.
In response to a question from the SDLP's Colum Eastwood, Ms Truss said she wanted to "work with all of the parties in Northern Ireland to get the executive and the assembly back up and running so we can collectively deliver for the people of Northern Ireland".
"But in order to do that we do need to fix the issues of the Northern Ireland Protocol which has damaged the balance between the communities in Northern Ireland," she said.
"I'm determined to get on with doing that and determined to work with all parties to find that resolution."
The executive collapsed in February after the DUP withdrew its First Minister Paul Givan over its objections to the protocol.
Ms Truss was then asked by former NI secretary Shailesh Vara if the government would press ahead with the NI Protocol Bill if it could not reach an agreement with the EU.
"We need to resolve the issue of the Northern Ireland Protocol," she replied.
"My preference is for a negotiated solution, but it does have to deliver all the things that we have set out in the Northern Ireland Protocol Bill.
"And what we cannot allow is for this situation to drift, because my number one priority is protecting the supremacy of the Belfast/Good Friday Agreement."
Mr Heaton-Harris called on the DUP to return to power sharing as soon as possible, but insisted he understood the importance of resolving issues with the protocol first.
He is expected to meet party leaders in Northern Ireland on Thursday.
Mr Heaton-Harris is the third Conservative MP to hold the NI secretary role within the past three months, and replaces Mr Vara who was sacked on Tuesday.
Mr Vara, who was in the role for 62 days, became the shortest serving Northern Ireland secretary ever.
Meanwhile, Steve Baker has replaced Conor Burns as minister of state in the Northern Ireland Office.
His appointment has just been confirmed by Downing Street.
Mr Baker was also a chair of the pro-Brexit European Research Group of Conservative MPs for several years.
In March, he called on the government to trigger Article 16 of the Northern Ireland Protocol, a mechanism to suspend parts of the deal.
🗣My speech to @EastBelfastCA last night ⬇️
The Northern Ireland Protocol was always unfinished business. That's why we said Boris' deal was a tolerable path to a great future.
⁰We surely now all agree the NIP does not deliver what it promised. https://t.co/q08ZatSrZg
The MP for Wycombe, he also co-chaired a group of MPs opposed to Covid restrictions.
New PM's team: Who's in and who's out
As it happened: Truss appoints more ministers on first full day as PM
New cabinet: Who is in Liz Truss's top team?
Hundreds held after Russia call-up sparks protests
Trumps 'inflated net worth by billions' – lawsuit
WATCH: Russian police carry away anti-war protesters. Video
The Russians risking freedom to protest Putin's war
The fake sex chats fuelled by an elaborate scam
Why Brazil's election matters so much to the US
Who are Iran’s morality police?
Fact-checking Russian claims Nato troops are in Ukraine
The vloggers capturing life on India's trains
16 moments from the Queen's 16 tours of Australia. Video
The men behind a record-breaking menstrual campaign. Video
The kids living in their parents' shadows
Four cities saying no to cars
The biggest myths of the teenage brain
The jobs employers can't fill
© 2022 BBC. The BBC is not responsible for the content of external sites. Read about our approach to external linking.