Irish Prime Minister Micheal Martin urged the UK government Friday to stand by its post-Brexit trade commitments in Northern Ireland, as Washington warned London its brinkmanship with Europe threatens peace.
Martin was visiting Belfast following anger on both sides of the Atlantic at a UK pledge to overhaul the so-called Northern Ireland Protocol, agreed as part of its Brexit divorce deal with the European Union.
Its requirement for checks on goods arriving from England, Scotland and Wales has infuriated unionists in Northern Ireland, who say it drives a wedge between the province and the rest of the UK.
They are refusing to join a new power-sharing government in Belfast until the issue is resolved, forcing its suspension.
"There is no substitute for a substantive series of negotiations between the European Union and the United Kingdom government in respect of solving issues in relation to the protocol," Martin told reporters in the Northern Ireland capital.
"We believe there can be a resolution of issues... but the only way to do that is through a negotiated settlement."
Hours earlier Nancy Pelosi, the speaker of the US House of Representatives, warned that the UK could forget about a free trade deal if it rewrites the protocol.
"It is deeply concerning that the United Kingdom now seeks to unilaterally discard the Northern Ireland Protocol, which preserves the important progress and stability forged by the (1998 Good Friday) Accords," which ended decades of bloodshed in the province, Pelosi tweeted.
"If the United Kingdom chooses to undermine the Good Friday Accords, the Congress cannot and will not support a bilateral free trade agreement with the UK."
A US Congress delegation jetted into Brussels on Friday and presented a united front against the UK with European Commission vice-president Maros Sefcovic.
"We're equally committed to protecting the Good Friday (Belfast) Agreement," Sefcovic tweeted. "Joint solutions implementing the Protocol are the only way to do so."
"The greatest strength between the United States and the @EU_Commission is our unity," wrote US congressman Richard Neal.
Britain says the protocol needs changing to end the political paralysis in Northern Ireland, and that its plans are a backup if talks fail with Brussels.
Martin earlier dismissed accusations that the EU was being inflexible and urged the biggest unionist party to rejoin the executive in Belfast.
Nationalists Sinn Fein are the biggest party for the first time in Northern Ireland's history.
Sinn Fein's Michelle O'Neill is set to become first minister in the executive. The position has equal power to the deputy first minister to ensure a balance of power between unionists and nationalists but her nomination is hugely symbolic as it has the post has never been held before by a pro-Irish nationalist party.
"The European Union has said repeatedly that we can move on issues," Martin told BBC radio.
He accused the Democratic Unionist Party (DUP) of holding the assembly to ransom by refusing to nominate ministers after recent elections.
"We can't have a situation where one political party determines that the other political parties can't convene in a parliament," Martin added.
But DUP leader Jeffrey Donaldson said the Brexit protocol had tipped the delicate balance in Northern Ireland that is needed for power-sharing to work.
"Power-sharing only works with the consent of unionists and nationalists," he told the BBC.
"There must be new arrangements if we are to move forward... the protocol has damaged both Northern Ireland's economic and democratic arrangements.
"It must be replaced by arrangements that can command the support of unionists as well as nationalists."
The protocol recognised Northern Ireland's status as a fragile, post-conflict territory that shares the UK's new land border with the European Union.
Keeping the border open with neighbouring Ireland, an EU member, was mandated in the Good Friday Agreement, given that the frontier was a frequent flashpoint during three decades of violence.
But it means checks have to be carried out elsewhere, to prevent goods getting into the EU single market and customs union by the back door via Northern Ireland.
The UK proposes creating a "green channel" for British traders to send goods to Northern Ireland without making any customs declaration to the EU.
US interest stems from the fact that it helped broker the Good Friday Agreement.
But Pelosi's warnings appeared to irk some in the UK's ruling Conservatives, as well as unionists in Northern Ireland.
"If Nancy Pelosi wants to see the agreement protected then she needs to recognise that it is the protocol that is harming and undermining the agreement and that is why we need to deal with it," Donaldson said.
