Italy’s far-right interior minister, Matteo Salvini, has compared African immigrants to slaves, prompting Luxembourg’s foreign minister to retort angrily using the French curse “Merde alors!”

The fiery exchange between Salvini – the leader of the League, which governs in coalition with the populist Five Star Movement – and Jean Asselborn occurred during a behind-closed-doors session at a European conference on security and immigration in Vienna.

Salvini, who immediately launched a crackdown on illegal immigration when his coalition government came to power, proudly published a video of the interaction on his Facebook page, alongside the post: “The Luxembourg [foreign] minister declared this morning that we need migrants because Europe is ageing. I told him I instead work towards young Italians (and Europeans) returning to bringing children into the world, because we don’t want new slaves.”

In the video, Salvini speaks about “being paid by citizens to help our young people start having children again in the way they did a few years ago, and not uproot the best of the African youth to replace Europeans who aren’t having children any more. Maybe in Luxembourg there’s this need; in Italy there’s the need to help our kids have kids, not to have new slaves to replace the children we’re not having.”

Asselborn, sitting two seats away from Salvini, gets increasingly agitated as the minister speaks, before interrupting in French with “allez, allez, allez”.

Salvini then says: “Your opinion is not mine. Maybe in Luxembourg there’s a need for new immigration but in Italy there’s a need to help people have children.”

The final remark triggered an angry response from Asselborn, who reminded Salvini of the Italians living in Luxembourg.

“They came as migrants and worked in Luxembourg so that you in Italy had money to pay for your children,” he said, before concluding: “Merde alors!”

“Merde alors” literally means “shit then”, but is closer to the English phrase “fucking hell”.

The exchange came as Filippo Grandi, the UN high commissioner for refugees, told reporters in Rome that he advised Salvini to tone down his aggressive language towards immigrants during a meeting earlier in week.