ROME—Premier Enrico Letta has harshly criticized a top Italian senator who likened the country’s first black cabinet minister to an orangutan, the latest episode of high-profile racial tension in a nation grappling with immigration.

In a statement Sunday, Letta denounced Roberto Calderoli’s words as “unacceptable” and “beyond every limit.”

Calderoli, the Senate’s vice-president and a leader of the anti-immigrant Northern League party, made denigrating remarks about Immigration Minister Cecile Kyenge while he was speaking at a party rally Saturday in northern Italy, the populist movement’s power base.

“When I see images of Kyenge I cannot help think, even if I don’t say that she is one, of a resemblance to an orangutan,” Corriere della Sera newspaper quoted Calderoli as saying. On Sunday, Calderoli said he was making a joke, and meant no offence to the minister.

Kyenge is a Congolese-born doctor who became Italy’s first black minister when Letta’s cabinet was sworn in in April. Reactions to her appointment have added to political tensions in Italy this summer, and Letta’s coalition government, which faces economic and other pressures, is extremely fragile.

Calderoli told the rally that Kyenge has done well to become a minister, but “perhaps she should do it in her own country.”

The Northern League isn’t in the government but has long been the closest political ally of former premier Silvio Berlusconi’s centre-right party, which is Letta’s main partner in the coalition government. Calderoli’s remarks sparked calls for him to resign, including from Public Administration Minister Gianpiero D’Alia.

D’Alia, a centrist, told Sky TG24 that Calderoli’s comments recalled the “language of the Ku Klux Klan.”

Kyenge said politicians should take the occasion to “reflect on what kind of debate they want . . . about content or about insults.”

Last month, Kyenge, who has lived in Italy since 1983, received death threats before she visited the northern region that is Calderoli’s party base. The xenophobic Northern League expelled a local politician after she suggested on Facebook that someone should rape Kyenge so she “can understand what victims of atrocious crimes feel.” The League’s leaders blame immigrants for violent crime in Italy.

In May, Mario Borghezio, a European parliamentarian for the Northern League, warned in a radio interview that Kyenge would try to “impose tribal traditions” on Italy.

Northern League secretary Matteo Salvini said Calderoli’s orangutan remark was a “shocking wisecrack, an out-of-line” comment.