WASHINGTON — Nearly a year after being told to do so, the head of the Environmental Protection Agency said Tuesday he couldn't say when he would comply with a Supreme Court directive and determine whether greenhouse gas emissions from vehicles should be regulated.

In a tense exchange with a senator, EPA Administrator Stephen L. Johnson suggested that few if any people at the agency were directly working on the issue now. The high court in April 2007 had said the EPA was required to determine whether carbon dioxide and other heat-trapping greenhouse gases posed a danger to public health.

Johnson originally had promised a reply to the court's ruling by last fall. Sen. Dianne Feinstein, D-Calif., pressed the EPA official at a hearing, repeatedly asking him how many EPA employees he had working on the greenhouse gas issue.

"Is anyone working on this at the present time, Mr. Johnson?" she asked. "How many members of your staff are currently working on this?"

"I don't know the answer to that," Johnson replied at a hearing of the Senate Appropriations environment subcommittee. Feinstein accused Johnson of "stonewalling" and said she found it strange that the EPA chief "can't give me a number (of people engaged) on something that is a Supreme Court finding."

"Madam Chairman, I am not stonewalling," Johnson said.

He said a law that Congress passed in December requiring automakers to achieve a fleetwide average of 35 miles per gallon by 2020 has complicated the EPA's response on greenhouse gas regulation as required by the high court.

Johnson also used the new auto fuel economy requirements as a key reason that he decided to reject a request by California for permission from the EPA to pursue its own controls on greenhouse gas emissions — mainly carbon dioxide — from automobiles.