By Adam Jourdan SHANGHAI (Reuters) - North Korean forces have seized a Chinese fishing boat, Chinese officials told state-run news agency Xinhua late on Sunday, creating a potential new irritant in ties between the two allies. Chinese counselor to North Korea Jiang Yaxian said North Korea had "grabbed" the private vessel from the northern city of Dalian in waters between China and the Korean peninsula, according to the official news portal. Tensions have been mounting between North Korea and China, its most important economic and political backer. ...