"We have formally written to Brussels to ask for the recall of (EU) ambassador (Saskia de Lang), who is engaging in activism activities that are contrary to the Vienna Convention," a Congolese foreign ministry official told AFP on condition of anonymity.
The letter, signed by Foreign Minister Jean-Claude Gakosso, was sent on May 9, the official said.
In its response on May 17, Brussels expressed its unwavering support for the EU representative, warning that "any decision from your side with regards to her will affect the relationship of trust between the EU and Congo", an EU spokesman in Congo said.
Sassou Nguesso's win only became possible after an October constitutional referendum ended a two-term limit on presidential mandates, allowing the former paratrooper to run again in a move denounced by his opponents as "a coup".
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Congo has been on edge ever since, with deadly clashes erupting in the capital that pitted government forces against suspected rebels.
After the vote, Mogherini denounced "the grave shortcomings of electoral governance" in Congo.
"This brings into question the credibility of the results."
The United States also expressed dismay in April at the conduct of the elections.