German European People's Party (EPP) chairman Manfred Weber | Gerard Julien/AFP via Getty Images Manfred Weber to Boris Johnson: Don’t talk Turkey German MEP warns Britain not to interfere in EU talks with Ankara.

Ahead of a European Parliament debate on Europe's fraying relations with Turkey, Manfred Weber, head of the European People's Party MEPs, voiced dismay Tuesday over British Foreign Secretary Boris Johnson's offer to assist Ankara in its negotiations to join the EU.

Weber urged Johnson and the U.K., which has voted to leave the EU, to butt out of the discussions with Turkey.

"This is unbelievable," Weber said in Strasbourg, where the Parliament is meeting in plenary session. "It is an arrogant provocation from Johnson where he's telling us what we have to do."

The debate in Parliament Tuesday afternoon is expected to focus in part on demands by Weber's center-right faction to end the negotiations with Turkey over its potential membership. Relations between the EU and Turkey have become strained in part because of the crackdown by Turkish President Recep Tayyip Erdoğan following a coup attempt in July.

Erdoğan has taken numerous steps to limit press freedoms and curb political opponents, including by carrying out wide-scale arrests. Last month he announced that the Turkish parliament would consider restoring the death penalty.

"The accession negotiations with Turkey should be frozen immediately," the EPP group wrote in a paid advertisement in POLITICO's Playbook on Monday.

Weber urged the U.K. not to interfere in EU-Turkish relations. “I ask the British government not to influence these discussions which will go on for the next two-and-a-half years. Please step back, it’s a question of fairness,” he said Tuesday.

Erdoğan has previously accused Brussels of acting in bad faith, expressing doubts about the commitment of European leaders toward eventual membership for Turkey in the EU. At the same time, the EU and Turkey are partners in an agreement on the return of migrants who have fled to Europe — a deal that was tied to billions in aid for Turkey. A serious rupture in relations could upend that agreement with consequences uncertain for both sides.

Maïa de La Baume contributed to this article.