The New York City Police Department, the Coast Guard and five other agencies boarded ships in New York Harbor Tuesday as part of a drill aimed at stopping vessels that could be carrying radioactive materials.

The exercise took place in the waters south of the Verrazano Bridge and was designed to simulate a multi-agency response using shared resources and expertise.

"It is one thing to do an exercise checking for nuclear radiological material on roadways. It's more challenging, certainly, to do it on the waterways," said Police Commissioner Ray Kelly. "I want to stress that everyday, the NYPD monitors for radiation on our waterway."

The drill is part of the ongoing Securing the Cities initiative, which is funded by the Department of Homeland Security. The goal of the program is to guard against the infiltration of a dirty bomb or nuclear device into the city.

"We practice together so that in the event that we have to come out here and do it for real, we have the equipment, the training and the familiarization with each other," said Assistant Chief James Waters of the NYPD Counterterrorism Bureau.

The future of the program remains uncertain as funding has been cut for next year. Authorities say the initiative has been integral in helping agencies coordinate among themselves to prepare for a terrorist attack.

Law enforcement officials say Tuesday's exercise was not in response to a specific threat.