Portugal has tightened a law that grants citizenship to descendants of Sephardic Jews and which allowed Russian oligarch Roman Abramovich to become a citizen, but the changes will not be retroactive, the foreign minister said on Wednesday.

The billionaire was granted Portuguese citizenship last year based on the 2015 law that offers naturalisation to descendants of Sephardic Jews who were expelled from the Iberian peninsula during the mediaeval Inquisition.

Foreign Minister Augusto Santos Silva said a decree introduced a new requirement for applicants to show an "effective connection with Portugal", although he made no mention of any individuals when discussing the alterations.

Abramovich was among several Russian billionaires added to a European Union blacklist, drawn up after Russia's invasion of Ukraine. He was put on a British sanctions list last week.

Santos Silva said Portugal would implement EU sanctions against Abramovich but could not ban him entering the country because he was a citizen.

The alterations to the citizenship law had already been given the green light by President Marcelo Rebelo de Sousa and would come into effect soon, the minister told a briefing for foreign journalists, without giving a date for implementation.

Portugal's Publico newspaper reported that, under the new rules, applicants would have to provide additional documents to show a connection to Portugal, such as inheritance of a property in Portuguese territory or proof of visits the country.

Santos Silva said the law was "generous and fair" and introduced for descendants of Jews who were persecuted in the region or expelled, although he said improvements were needed.

An inquiry into the granting of citizenship to Abramovich was opened in January by the Portuguese prosecutor's office and his citizenship could be stripped depending on its outcome.

A spokesperson for Abramovich has repeatedly denied any wrongdoing.

A rabbi responsible for issuing a document needed to obtain citizenship was arrested last week over suspicions of money laundering, corruption, fraud and falsification of documents in the process.

Close to 57,000 descendants of Sephardic Jews have been granted citizenship since the law was implemented in 2015, according to official data.