(CNN) Europe has told British Prime Minister Theresa May, again, that it will not reopen talks on the Brexit agreement as she arrived in Brussels to persuade EU leaders to agree to key changes as demanded by the UK Parliament.

European Commission President Jean-Claude Juncker said Thursday the EU could only "add wording" to the Political Declaration about what they aim for in their future relationship.

President of the European Commission Jean-Claude Juncker greets UK Prime Minister Theresa May in Brussels on Thursday.

May's meeting with EU leaders takes place amid a frosty atmosphere after European Council President Donald Tusk tweeted there was a "special place in Hell" for people who "promoted Brexit" without a plan to execute it. His words rattled politicians in the UK and drew sift condemnation from Brexiteers.

I've been wondering what that special place in hell looks like, for those who promoted #Brexit, without even a sketch of a plan how to carry it out safely. — Donald Tusk (@eucopresident) February 6, 2019

After meeting Tusk on Thursday, May said the language he used in the tweet "was not helpful and caused widespread dismay in the United Kingdom."

British lawmakers voted last month to send May back to Brussels to renegotiate the terms of the Brexit deal, specifically over concerns regarding the Irish backstop. The backstop -- an insurance policy designed to avoid a hard border between Northern Ireland, which remains part of the UK, and the Republic of Ireland, an EU member -- has been a particularly thorny issue in May's deal, with British politicians firm that they will not back her deal without changes.