GENEVA – Four diamonds worth millions were stolen on Wednesday from the world's biggest watch and jewelery fair in the northern Swiss city of Basel, the local prosecutor's office said.

"On Wednesday, 30 March 2011, at around 11:00 am (0900 GMT), one of the as yet still unidentified perpetrators distracted an employee of a diamond trader in the exhibition hall 3.0, and stole four diamonds worth millions from the display window," said Markus Melzl, spokesman of Basel's prosecutor's office.

After the robbery was detected, the exhibition hall was closed for half an hour to prevent the thieves from escaping, but to no avail.

"Apparently, the perpetrators successfully left the hall with the stolen goods," said Melzl in a statement.

The spokesman told AFP that the theft involved "probably four to five accomplices."

"Three diverted the attention of the diamond trader's employees, while two others opened the showcase," he said.

The thieves are believed to have checked out the display showcase several days before the theft, allowing them to make a quick move.

Melzl noted that the Israeli trader, who was not identified, only realised the theft about 15 minutes after it was carried out, allowing sufficient time for the perpetrators to get away.

"It was carried out in a very professional manner," he added.

Police are examining closed-circuit camera footage and looking into possible DNA traces and fingerprints.

Some 1,892 exhibitors from 45 countries, including luxury brands such as Chopard, Patek Philippe, and Rolex as well as jewelery makers like Bulgari and Tiffany, are showcasing their latest creations over 160,000 square metres at the Baselworld fair until Thursday.

The fair, which is open to the public, has been the targeted by thieves on several occasions. In 2009, a robber stole 13 million francs (10 million euros) of jewelery, but was rapidly caught.

In 2007, thieves made away with more than a million francs worth of jewelery.