(Updates with union statement)
May 21 (Reuters) - Westinghouse Electric Co, a unit of Japan's Toshiba Corp, said on Sunday it issued a lockout notice to 172 union members at its nuclear components manufacturing plant in Newington, New Hampshire, declaring that the sides had reached a stalemate in contract negotiations.
Westinghouse, which has been operating under bankruptcy protection, began formal negotiations with the International Brotherhood of Boilermakers, Iron Shipbuilders, Blacksmiths, Forgers and Helpers in April.
The Boilermakers and Westinghouse engaged in a mediation process that was not successful, and the union has refused to accept the company's last, best and final contract offer, the company said.
"As the Boilermakers were not willing to accept the offer, the company made the difficult decision to invoke a lockout," Michele DeWitt, Westinghouse interim senior vice president for nuclear fuel & components manufacturing, said in a statement.
The lockout was set to go into effect just before midnight on Sunday.
A union spokesman said the company and union had held only five negotiating sessions and one meeting with a mediator from which the company walked out.
"Clearly, Westinghouse has not bargained in good faith and we will be exploring all available recourses against the company, including filing unfair labor practice charges with the National Labor Relations Board," spokesman Tyler Brown said in an emailed statement. (Reporting by Bill Berkrot; Editing by Sandra Maler and Andrew Hay)