It's not an invasion.

Authorities are reassuring worried residents that those mysterious helicopters flying low over parts of the Twin Cities metro area Monday night aren't anything menacing.

The U.S. Special Operations Command has been conducting routine urban-environment training in Minneapolis and Saint Paul since Aug. 19, supported by police in both Minneapolis and St. Paul.

The training, which had been conducted in locations not easily seen by the public, became more visible as of Sunday, when helicopters became part of it. Residents can expect to see or hear low-flying military transport helicopters, specifically Black Hawks and smaller Hughes 500s between 7 p.m. and midnight.

Police and news outlets received several calls about the helicopters Monday evening.

All flights and flight routes have been coordinated with local authorities and the Federal Aviation Administration, authorities said. Exact training times and locations are not being released to prevent people from gathering to attempt to watch the training and creating a safety hazard.

The operations will continue until Saturday.