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 organised 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.

However, the court denied the suspects bail.

Botezatu's name did not come up during that hearing.

Eugen Darie told the court he visited Rotterdam Kunsthall museum at around the time of the theft last October but never laid eyes on the stolen paintings he and his co-defendants allegedly tried to sell.

"I didn't see the paintings," he said. "I only looked at bronze statues. I am innocent."

Fellow suspect Radu Dogaru also told the court he had nothing to do with the heist, one of the most spectacular in the art world in the last 20 years, even though prosecutors say he was present when two of the works were offered to a Romanian businessman in the presence of an art expert.

Mihai Alexandru Bitu, the third suspect, also denied involvement.

His lawyer, Daniela Dede, told AFP that her client "just received a call from his co-defendant Dogaru who asked him to find a buyer for some objects. He didn't know it was these paintings."

The suspects, all in their 20s, have been detained for questioning since their arrest on January 22. Under Romanian law, they can be held for 29 days.

The heist gripped The Netherlands and the art world as police apparently struggled to piece the crime together, despite putting 25 officers on the case.

Dutch police released grainy security camera footage of the theft, which took place around 3:00 am. The footage showed two apparently young males entering and leaving the museum in central Rotterdam within barely 90 seconds.

The works stolen include Picasso's "Tete d'Arlequin", Monet's "Waterloo Bridge" and Lucian Freud's "Woman with Eyes Closed".