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Senator Chris Murphy, a member of the US Foreign Relations Committee, also said he would not support any bilateral trade agreement between the US and the UK while outstanding issues with the post-Brexit protocol still remained to be resolved.
The Democrat senator is in Northern Ireland where he met with political leaders at Stormont.
The UK and the EU have been locked in negotiations over the implementation of the protocol, which is designed to prevent checks along the Irish border.
The UK Government has threatened to trigger Article 16 of the protocol, which would suspend parts of the agreement and risk a major escalation in tensions with the EU, if solutions to outstanding trading issues cannot be resolved.
This is a critical moment for Northern Ireland as we are watching the negotiation play out over the future of the protocol
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Senator Chris Murphy
But Mr Murphy said that invoking Article 16 would be to the “detriment” of people in Northern Ireland.
Speaking at Stormont, he said: “I am here on behalf of Republicans and Democrats in the United States Senate who are interested in continuing our legacy of friendship with Northern Ireland.
“This is a critical moment for Northern Ireland as we are watching the negotiation play out over the future of the protocol.
“We want to make sure that the protocol is moved forward in a way that benefits all of the people of Northern Ireland.
“To my mind, it would not be good for Northern Ireland if Article 16 was invoked, which could ultimately provoke a crisis, a trade war which would act to the detriment of the people here.”
He added: “This is a negotiation between Britain and the European Union, but the United States, having been an important player in the Good Friday Agreement, wants to make sure that nothing happens with respect to Britain’s departure from the European Union that compromises peace here in Northern Ireland.
“My visits here in Belfast and tomorrow in London are intended to demonstrate that there are Republicans and Democrats in the Congress that want to make sure that negotiations over the protocol are conducted in a way that does not compromise the Belfast/Good Friday Agreement.
“I think that something can be worked out by the end of this year, but it will likely involve the United States pushing a little bit, both on London and on Brussels.”
Asked about a trade deal between the US and the UK, Mr Murphy said: “I don’t think it would be a good idea for the United States to move forward with a bilateral trade agreement with Britain if there are outstanding questions about the future of the Belfast/Good Friday Agreement.
“So long as there is uncertainty about the trade relationship between Britain and the European Union, so long as the negotiation over the protocol is still outstanding, then, to my mind, the Belfast/Good Friday Agreement is potentially compromised.
“I have made clear that I am not going to support a free trade agreement between the United States and Britain, and Congress would have to sign off on that agreement, until we are absolutely certain that Brexit and the negotiations between Britain the European Union are concluded.”
Mr Murphy will be in London on Tuesday where he will meet with Brexit minister Lord Frost.
DUP leader Sir Jeffrey Donaldson met with the senator on Monday, and said that he told him that removal of the Irish Sea border is “fundamental to protect the foundations of political progress in Northern Ireland”.
Sir Jeffrey added: “Over the weekend, Mr Murphy and Pennsylvania Senator Pat Toomey wrote to the Prime Minister outlining their support for the Belfast Agreement. Central to the Belfast Agreement was the principle of consent, something which the Northern Ireland Protocol fundamentally undermines.
“You can have the Belfast Agreement or you can have the protocol, you cannot have both.”
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