Federal officials have put a freeze on Boston’s new tactic of killing rats by using dry ice.

City workers began using the super-cold substance to exterminate rats earlier this year, packing it into rat burrow exits and letting it melt. Dry ice, which is frozen carbon dioxide, reverts to a gas as it melts, filling the burrows and suffocating rats inside.

Boston officials said the method is effective, more humane, and significantly cheaper than using rat poison.

“We were using it with great success,’’ said William Christopher, commissioner of the city Inspectional Services Department, adding the city had not run into any problems during the new campaign.

But the US Environmental Protection Agency has ordered Boston to stop the practice — at least for now.

The EPA said Boston must register carbon dioxide as a pesticide used for rodent control with federal and state agencies.

The EPA’s ruling came after state officials asked the federal agency for guidance.

Christopher said city officials have stopped using dry ice, a development first reported by NECN, and are working to fulfill the registration requirements.

He said he hopes to get the green light to resume using dry ice by the spring, when he said rat activity tends to pick up.

“Luckily, right now, it’s during the winter season, so it’s slow in terms of rodent activity,’’ he said.

In the meantime, Boston officials have resorted to using their old standbys for rat control: traps and poison, Christopher said.

Matt Rocheleau