The former US Special Envoy to Northern Ireland, Gary Hart, has written to the US Senate expressing his concerns over the impact of Brexit on the peace process.
In a letter to the Senate Foreign Relations Committee, Mr Hart said he was urging members to make clear to the UK and European Union the absolute necessity of protecting the Good Friday Agreement by maintaining an open border.
He is the last person to occupy the role as it has been left vacant since Donald Trump took office.
In his letter to US senators, Mr Hart wrote that there was great concern over the possibility of the restoration of a hard border between Northern Ireland and the Republic of Ireland.
"Virtually all knowledgeable observers believe such a policy has the distinct possibility of reigniting sectarian conflict and violence and thus abrogating the central purpose of the Good Friday Agreement," he wrote.
Mr Hart urged members of the US Senate to continue to build on the strong bipartisan consensus, which he said had helped to sustain the Good Friday Agreement for the last 20 years.