Thousands of Japanese "yakuza" mobsters have officially formed a new group after splitting off from the country's largest organised crime syndicate, raising concerns for possible inter-gang conflicts, reports said Sunday.

Leaders of 13 factions kicked out of the "Yamaguchi-gumi" syndicate -- Japan's largest mobster or "yakuza" group which boasts 23,000 members and associates -- held the first formal meeting on Saturday in the western city of Kobe, local media reported.

One of the expelled factions is known as the Yamaken-gumi, which has about 2,000 members and whose head was chosen to lead the spin-off, according to the Yomiuri Shimbun newspaper.

The rival group is expected to have a total membership of about 3,000, the Yomiuri said.

Like the Italian Mafia and Chinese triads, the yakuza engage in everything from gambling, drugs and prostitution to loan sharking, protection rackets and white-collar crime.

But under Japanese law membership of the groups, which also conduct legitimate businesses, is not illegal and each of the designated groups have their own headquarters.

As fears rise that the split could lead to a wave of gang violence, Japanese police last week called for nationwide vigilance, holding an emergency meeting with special officers from the country's 47 prefectures.