Derek Chollet told the BBC the US hoped the row over Northern Ireland’s post-Brexit trade deal could be resolved.
He said “a big fight between the UK and the EU” was “the last thing” Washington wanted.
Vladimir Putin would “use any opportunity he can to show that our alliance is fraying”, he added.
Mr Chollet, the most senior adviser to US Secretary of State, Anthony Blinken, said: “We want to see this issue resolved and we want to see the temperature lowered and no unilateral acts.
“And it’s particularly important right now where we need to send a message of unity to the world and not undermine all the things that we’ve been so successful in working on together over the last several months and showing unity in Ukraine.”
Mr Chollet’s intervention is significant. It is rare for senior US officials to comment on the UK’s domestic affairs given the historically close relationship between the two nations.
But his comments build on recent concerns expressed by senior US politicians, including President Joe Biden and US House Speaker Nancy Pelosi, about the UK government threatening to override the Northern Ireland Protocol.
‘Bigger fish to fry’
These concerns have so far largely focused on what the US sees as the possible risks to peace in Northern Ireland.
But the senior State Department official is making a new argument, that now is not the time for the UK and the EU to be having a fight. There are bigger fish to fry over Ukraine.
The US wants its allies united, not scrapping over legacy Brexit issues.
- The Northern Ireland Protocol is part of the Brexit deal: it means lorries don’t face checkpoints when they go from Northern Ireland (in the UK) to the Republic of Ireland (in the EU)
- Instead, when goods arrive in Northern Ireland from the rest of the UK (England, Scotland and Wales), they are checked against EU rules
- The UK and the EU chose this arrangement because the Irish border is a sensitive issue due to Northern Ireland’s troubled political history
On Thursday Ms Pelosi said she was “deeply concerned” that the UK was seeking to “unilaterally discard” Northern Ireland’s post-Brexit trade arrangements, set out in the protocol.
She said the US Congress would not support a trade agreement with the UK if its actions jeopardised the peace process in Northern Ireland.
The UK government has argued that changes to the way goods are shipped from Great Britain to Northern Ireland are needed to restore its devolved government.
A power-sharing administration cannot be formed without the Democratic Unionist Party (DUP), which has refused to join one until significant changes are made to the protocol.
The DUP says the protocol, which was agreed by the UK and the EU in December 2020, has created economic barriers between Great Britain and Northern Ireland.
On Tuesday Foreign Secretary Liz Truss said the UK government would introduce a law to change the protocol unilaterally should negotiations with the EU fail.
Ms Pelosi warned against any action that might endanger the Good Friday Agreement, the peace deal that ended decades of conflict in Northern Ireland.
In the first of several tweets, she wrote: “Ensuring there is no physical border between the Irish Republic and Northern Ireland is necessary for upholding this landmark agreement, which transformed Northern Ireland.”
But the UK’s former Brexit minister, Lord Frost, has criticised Ms Pelosi for making what he called an “ignorant” statement about the situation in Northern Ireland.
Lord Frost, who negotiated the protocol with the EU, said there was no plan to put a physical border in place on the island of Ireland.
“Nobody’s ever suggested that. So I don’t know why she’s suggesting that in her statement,” Lord Frost told the Week in Westminster on BBC Radio 4.
He also denied that making changes to the arrangements would undermine the Good Friday Agreement.
“It is the protocol itself that’s undermining [the Good Friday Agreement] and people who can’t see that really shouldn’t be commenting on the situation in Northern Ireland,” said the former minister.
On Friday afternoon DUP leader Sir Jeffrey Donaldson said he thought Ms Pelosi’s contributions were “entirely unhelpful, offer no solution, offer no help and merely repeat a mantra that frankly is hopelessly out of date.”
He spoke as the Irish Taoiseach Micheál Martin met Northern Ireland’s main parties to discuss the protocol and the political crisis.
Meanwhile, a US Congress delegation flew to Brussels for a meeting with European Commission vice-president Maros Sefcovic, who has been leading negotiations with the UK.
“We’re equally committed to protecting the Good Friday (Belfast) Agreement,” he tweeted. “Joint solutions implementing the Protocol are the only way to do so.”