The EPA expressed concern about the level of greenhouse gas emissions tied to the production of tar sands oil, the type designated for the Keystone XL pipeline.
Howard said in a statement the "scope and tone" of the EPA statements were "somewhat surprising."
"There are no 'new issues' identified in their letter," he said.
Howard said the EPA's concerns over emissions fail to take into account the amount of Venezuelan and Mexican crude oil that would be displaced by Keystone XL.
The U.S. State Department review took note of rail options for crude oil delivery. Howard said that more oil was shipped by rail during the first half of last year than the designed capacity of Keystone XL. Rail deliveries produce "triple" the emissions than pipelines, he said.
"TransCanada is conducting a detailed technical review on information put forward in the EPA letter," he said. "Once that review is completed, a detailed response will be sent to the Department of State and will be published."
There was no corresponding defense from the EPA.