MADRID- The Spanish authorities have identified assets, including houses, companies and at least one luxury yacht, belonging to 15 Russian oligarchs on the EU sanctions list, an official overseeing property registries told Reuters on Thursday.

Mariano García Fresno, the head of the General Council of Notaries' money laundering prevention unit, said the unit had detected activity after the Feb. 24 invasion of Ukraine as the oligarchs sought to transfer their stakes in some companies to relatives or associates to avoid detection.

"They were already starting to move these holdings, especially in Spanish companies, or holdings or shares in foreign companies," García Fresno said.

"We have located some 15 people from that (EU sanctions) list and another 105 people linked to them or family members (...) There are people who are less well known on the list, but some of them were among the best known."

Every week since Russia invaded Ukraine, the European Union has introduced new sanctions on Moscow, adding hundreds of top politicians, businessmen and military staff to its blacklist.

Implementation, however, faces legal constraints and difficulties in identifying the real owners of assets, such as real estate, yachts or bank accounts.

This week, the European Commission, the EU executive, said it would introduce a legislative proposal to prosecute those who attempted to evade sanctions, for instance by transferring their assets to family members.

After reviewing more than 1,000 names among millions of transactions registered with Spain's public notaries since 2004, García Fresno's unit established that sanctioned oligarchs had stakes in at least 10 Spanish companies and 13 foreign ones.

The information was handed over to the country's financial intelligence unit and the Treasury and, in late April, the government confirmed it had frozen 12 funds and bank accounts linked to five people on the sanctions list, along with three luxury yachts and 23 properties.

Companies linked to Russian oligarchs own some 35 real estate properties in Spain, including several luxury homes, García Fresno said, as well as bank accounts and stakes in companies operating in Spain and elsewhere in Europe.

His unit found that in Spain foreign companies owned the companies that owned assets linked to the oligarchs, but even though there were two or three intermediaries, the prevention unit was able to track their ownership through databases.

(Reporting by Corina Pons; editing by Barbara Lewis)