GOP presidential primary frontrunner Mitt Romney added more uncertainty over his tax plan Wednesday morning, citing that any private analysis of it was inaccurate because it 'can't be scored" without all the details "laid out."

Romney appeared on CNBC's Squawk Box and was asked if his tax proposal, which would lower rates on the highest tax bracket from 35 to 28 percent and cut income tax rates by 20 percent, would add several trillion dollars to the deficit.

"I think it's kind of interesting for the groups that try and score it because frankly it can't be scored," Romney said. "Because those kinds of details are going to have to be worked out with Congress, and we have a wide wide array of options."

Despite the former Massachusetts governor's requests, private groups have estimated that Romney's current proposal would cost $10.7 trillion over 10 years, four times the cost of the Bush tax cuts. University of Connecticut Professor James Kwak called Romney's tax plan a "mathematical disaster," saying the former Massachusetts governor's claim of his plan being deficit neutral with major spending cuts is faulty.

The Committee for a Responsible Federal Budget said that Romney's plan would cause a $2.6 trillion deficit if it's not paid for.

WATCH: Video from CNBC, which was broadcast on March 7, 2012. (Highlighted mark from 14:04 to 14:28)