Washington ? Critics of a plan to send undercover air marshals to patrol trains, buses and ferries say it will take scarce resources away from airplanes and could get in the way of local police.

The Transportation Security Administration, in a little-noticed announcement on Tuesday, said teams of air marshals would begin counterterror surveillance on land as part of a small test program that also involves bomb-sniffing dogs and transit inspectors.

Beginning Wednesday, the teams were going out for three days at a bus station in Houston and at rail facilities in Baltimore, Washington, Los Angeles, Atlanta and Philadelphia.

American Airlines pilot Denis Breslin, spokesman for the airline’s pilots’ union, said air marshals ought to stick to airplanes.

“I don’t think there’s enough air marshals to cover commercial aviation as it is,” Breslin said. “That’s what transit police are for.”

Homeland Security Secretary Michael Chertoff, speaking in New York, said that aviation continues to be a priority.