April 12 (Reuters) - The United Auto Workers (UAW) said on Friday it is "extremely disappointed" with truck maker Volvo Group's decision to set up a new truck plant in Mexico, which the company said would help supplement its U.S. production.

The union, in a letter published on Thursday, said it is looking into the contractual obligations for truck build rates at the Lehigh Valley Operations (LVO) plant and the potential legal ramifications from Volvo's Mexico decision.

Volvo announced the new plant on Thursday and added that it would be investing an additional $80 million for future production at the LVO plant, having invested $73 million over the last five years.

"This is a slap in the face for all of us," said UAW Local 677 shop chair Tim Hertzog, adding that the company informed the union about the plans 15 minutes before the announcement.

Volvo was not immediately available for a comment on Friday. (Reporting by Abhinav Parmar in Bengaluru; Editing by Ravi Prakash Kumar)