Theresa May has said Europe has agreed that there will be no physical border on the island of Ireland after Brexit.
The British Prime Minister said it was vital that the UK's split from the EU has no impact on the Northern Ireland peace process.
At the close of her involvement in the European Council summit in Brussels, Ms May said the Good Friday Agreement must be at the heart of any deal.
The PM said: "Northern Ireland's unique circumstances demand specific solutions.
"It's vital that joint work on the peace process is not affected in any way. It's too important for that.
"Both sides agree that there cannot be any physical infrastructure at the border."
The PM said European leaders also gave commitments that the Common Travel Area between Britain and Ireland should be protected.
Earlier, Taoiseach Leo Varadkar revealed Ms May had made the British position clear on no hard border in her address at the European Council summit dinner in Thursday night.
The Taoiseach said he was heartened by the PM's strengthened language on the issue.
"She specifically referenced the unique situation for both Ireland and Northern Ireland, which I think was very positive," he said.
"She strengthened her language in relation to the border.
"She said the UK would not accept a physical border on the island of Ireland - again, very positive language."
The Taoiseach said talks on the border question were "inching forward" but he repeated his call for more detail.
"We don't have more detail yet - in a sense that's part of the difficulty," he said.
"We need to see sentiment backed up."
Brexit will dominate day two of the summit, which involves all EU leaders bar Ms May. Mr Varadkar also said the question of the UK's divorce bill is not as big an issue for Ireland as it is for other countries.
He said his focus was on citizens' rights on the island of Ireland, Europeans living in Britain and British citizens in Ireland along with the question about of future UK-Ireland trade.
The European Council said Brexit chief negotiator Michel Barnier would be asked to pursue further refinement of principles on protecting the Good Friday Agreement and the Common Travel Area.
In its closing statement, it said this would have to take into account avoidance of a hard border, but it placed the onus on the UK to present and commit to "flexible and imaginative" solutions.