A top German diplomat urged the U.K. to honor the post-Brexit Northern Ireland Protocol, and warned that the dispute over the issue is “not a game” as European Union ministers rallied behind the bloc’s chief negotiator.

“Dear friends in London and in Britain, please come to your senses,” Deputy Foreign Minister Michael Roth said in Brussels on Tuesday ahead of a meeting with EU counterparts. “Following painstaking negotiations, which lasted for a very long time, we made clear agreements.”

The protocol allowed the U.K. to leave the EU’s single market without creating a hard border on the island of Ireland. Under the deal, goods moving into Northern Ireland from the rest of the U.K. are subject to customs checks if they could potentially later be moved into the EU.

After threats by U.K. Prime Minister Boris Johnson’s government to renege on the deal, the two sides welcomed progress by the end of last week, signaling optimism a trade war can be averted.

EU ministers on Tuesday expressed strong support for their chief negotiator, Maros Sefcovic, who briefed them on the latest talks and reaffirmed the bloc’s refusal to renegotiate the protocol, according to two diplomats who asked not to be identified discussing a private meeting.

Neither Sefcovic nor member states specified how the EU might react if London suspends the protocol, as they all prioritize focusing on proposals to end the impasse made by the European Commission, the diplomats said.

Britain’s Brexit minister, David Frost, said on Friday that there was “potential to generate some momentum” in talks, as Sefcovic cited progress on how medicines would be handled.

“What we are going through here is not a game,” Germany’s Roth said in Brussels. “It’s about the interests of people in the U.K. and it’s about the interests of the European Union.”

“Damn it—we want to work together in a cooperative and friendly way,” he added. “We don’t want to constantly fight over what we’ve already agreed.”