Romanian police investigating the spectacular October theft of seven masterpieces from a Rotterdam museum searched the home of a fashion designer on Monday and arrested his assistant, a police source said.

They searched the home of Catalin Botezatu and were also "interrogating him in the framework of the inquiry," the source said.

His assistant Petre Condrat was detained on charges of receiving stolen goods, bringing to four the number of people arrested in Romania over the heist.

Botezatu, 46, who has taken part in several Fashion Weeks including New York, was questioned by the prosecutor's office dealing with organized crime, which would decide whether to charge him.

Three Romanians arrested last week in connection with the theft of the masterpieces, including works by Monet and Picasso, claimed their innocence in court on Friday.

The men, charged with conspiracy and aggravated theft, have been linked to at least two of the artworks - a Matisse and a Gauguin - which they allegedly tried to sell in Romania.

Their defence lawyers told the court at a bail hearing that they were innocent even though one of them twice visited the museum where the seven masterpieces, estimated at between 100 and 200 million euros ($135 million and $270 million), were stolen. The court denied the suspects bail.

The suspects have been detained for questioning since their arrest on January 22. Under Romanian law, they can be held for 29 days.