About 25 guards at Xcel Energy's Monticello nuclear power plant have been locked out by their employer, G4S, and no resolution to the labor impasse appears to be in sight.

Members of United Security Professionals Local 2, which represents "lieutenants" or "shift leaders" at the Monticello plant, were locked out at midnight Saturday when their contract expired. The union and G4S, a large global security firm, have been at odds for several months, and negotiations broke down earlier this summer.

"They haven't really expressed any interest in speaking to us," said Josh Haider, head of Local 2.

The union's primary concern is a G4S plan to move to an inferior health plan and cut its contributions to guards' health care premiums by nearly 50%, or $10,000 per worker annually, Haider said.

London-based G4S did not return requests for comment.

Minneapolis-based Xcel and G4S have been training replacements for Local 2 guards over the past month. Xcel, Minnesota's largest electricity provider, has said it provides training for all security contractors at its properties, a common power-industry practice.

Local 2 is one of two union locals of the independent United Security Professionals, which together represent around 120 guards at the Monticello nuclear plant. Local 1's contract at the Monticello plant does not expire until winter.

G4S took over Xcel's nuclear-plant security earlier this year. Before G4S, Securitas held Xcel's nuclear security contract for five years. Securitas had taken over the job from G4S.