The head of Korea's militant trade union said Tuesday his union has withdrawn support from a left-wing political party mired in infighting over alleged primary fraud.

"It is impossible for the Korean Confederation of Trade Unions to continue to support the Unified Progressive Party," Kim Young-hoon, president of the labor umbrella group, in an interview on local radio.

The trade union forged an alliance with the UPP and its predecessor in previous elections in an apparent bid to boost their chances of winning elections.

Kim said recent violent clashes between the party's rival factions at the UPP's central committee conference have raised a fundamental question over whether the party plays its political role.

However, he suggested his union members would not bolt from the party en masse.

The trade union is a key backbone of the five-month-old party as some 35,000 union members have voting rights in the party, accounting for 46 percent of party officials who are eligible to vote in the party's internal elections.

The union boasts about 700,000 members who work in vehicle plants, shipyards, subways and other public transportation systems, according to union spokesman Park Song-shik.

Kim's comment came a day after a non-mainstream faction set up an interim emergency committee to reform the minor party torn apart by the primary fraud. The move triggered a strong protest from a mainstream faction loyal to the East Gyeonggi Coalition, an alleged pro-North Korean bloc that some say was dissolved a decade ago.

The factional division has deepened since earlier this month when the party announced after an internal investigation that its primary to select the party's proportional representation candidates for the April 11 elections had been rigged.

Seven UPP candidates were elected through direct elections and six others won parliamentary seats under the proportional representation system, which allocates seats to parties according to the numbers of votes they receive.

The UPP, which was created just five months ago, has become the third-largest party in the 300-seat National Assembly. (Yonhap)