There is no “spirit of revenge” in the Brexit negotiations, Michel Barnier said.
He rejected DUP criticism of his handling of the talks but said only a Northern Ireland-specific solution to the border question will work.
He said: “There is no spirit of revenge.
“I personally regret Brexit for many reasons and I am never aggressive.
“My door is open.”
Businesses fear checks on goods and people would disrupt free-flowing cross-border trade, and some critics believe it could even threaten the peace process.
Mr Barnier is holding a two-day visit to both sides of the border and addressed a conference in Dundalk in Co Louth on Monday.
He said: “Only Northern Ireland-specific solutions will work, only this can address the challenges created by Brexit in full.”
Mr Barnier asserted there must not be a hard border on the island of Ireland and there was no choice but to protect the 1998 Good Friday Agreement which largely ended violence in Northern Ireland.
If a withdrawal agreement was to be found in October the framework of this agreement must be clear, the senior European representative warned.
Mr Barnier said a European Council meeting of leaders in June was a “stepping stone” to the final October summit when a Brexit deal must be sealed.
In response to criticism from pro-Brexit DUP leader Arlene Foster, Mr Barnier said his negotiating style was not aggressive.