"Under no circumstances will we accept that the EU wants to do harm to the Good Friday Agreement," they said.
"The purpose of our work is to protect it in all its dimensions."
Meanwhile a German government spokesman confirmed Merkel and Johnson had spoken on the phone this morning, however said it would not report from "confidential conversations".
The DUP  responded to the suggestion that Northern Ireland stay in a customs union as "beyond crazy".
In a statement released by the party, Arlene Foster, leader of the DUP, said that "no UK Government could ever concede such a surrender".
"The EU is not interested in a negotiated outcome at this time," she said.