The European Parliament's Brexit coordinator has said he got a "shock" on his recent visit to Belfast when he saw the city's peace walls.
Guy Verhofstadt described Northern Ireland as having a "frozen conflict", and said the Good Friday Agreement must not be damaged by Brexit.
He suggested that the agreement could be attached to the Brexit withdrawal deal as a way of protecting it.
Mr Verhofstadt made the comments in a debate at the European Parliament.
It was voting on a resolution assesses the state of play in the Article 50 negotiations between the UK and the EU.
MEPs backed a motion urging the EU not to open the next phase of the discussions until a "major breakthrough" has been made.
The resolution referred to Prime Minister Theresa May's recent statement about wanting no infrastructure on the Irish border after Brexit.
It stated that position "presumes" the UK will have to stay in the internal market and customs union, or that Northern Ireland will stay in some form of internal market and customs union.
That part of the text has been welcomed by Sinn Féin but criticised by unionists.
Ulster Unionist MEP Jim Nicholson said it showed that Mr Verhofstadt was "taking sides" in Northern Ireland.
"Mr Verhofstadt is essentially calling for an international border to be placed between Northern Ireland and the rest of Great Britain," he said.
Mr Nicholson added that it would be a "sad day" if the parliament supported that text as it would be "turning its back on decades of good relationships with all communities in Northern Ireland".
'Important step forward'
That was echoed by the Democratic Unionist Party's Diane Dodds, who said "there can be no question of a deal that cuts us adrift from our most important market and erects barriers in the UK single market".
Sinn Féin's Martina Anderson said the resolution was "an important step forward in recognising that the requirements of the north of Ireland are different from those in Britain".
She also welcomed Mr Verhofstadt's comments on exploring how the Good Friday Agreement could be given legal protection as part of the withdrawal deal.
Mr Verhofstadt was in Belfast last month, and said during that visit it was up to the UK to find a way to avoid the imposition of new controls at the Irish border after Brexit.
The parliament's role in the Brexit process is to vote on the final deal.
Its leaders are consulted by the EU's chief negotiator Michel Barnier, although he takes his direction from the European Council.