More than 7,000 unionized truck drivers went on strike today in South Korea, adding to the country’s recent shipping woes, according to a report from Korea’s Yonhap News Agency. Transport Minister Kang Ho-in called the walkout illegal and “an act of betrayal,” vowing swift action against participants. The trucking walkout comes during a prolonged strike by members of the country’s railroad and subway union, putting further pressure on Korea’s cargo shipping industry. As of Monday, 7,390 of the total 18,351 unionized railroad and subway workers were participating in the strike, which began two weeks ago, according to the ministry. With the latest strike, the Korean government estimates trucking volumes could drop as much as 32 percent from normal levels.