Updated

Sen. Tom Coburn is accusing the Treasury Department of withholding information on the millions of dollars the Internal Revenue Service blew on conferences between 2010 and 2012.

The Oklahoma Republican sent Treasury Secretary Jacob Lew a letter demanding to know why the department's response to a separate letter he sent a year earlier left out millions in expenses which have recently come to light in an inspector general’s report released Tuesday. The report said the IRS spent $50 million on conferences from 2010-2012.

But Coburn’s June 3 letter, obtained by Fox News, said then-Treasury Secretary Tim Geithner acknowledged only a small fraction of that spending in his response last year. Coburn called that response "inaccurate and incomplete."

Treasury responded to Coburn’s 2012 letter in which he asked for a full listing of all conferences attended by department employees during 2010-2012. The department sent him a list of conferences attended by 50 or more employees between Jan. 1, 2005-June 1, 2012.

“On that list, there were only five conferences, with a total cost under $500,000,” Coburn said. “Only one of these conferences took place during the time period requested in the letter. None were related to the IRS or its employees.”

On Tuesday, a report from the Treasury inspector general for tax administration showed that the IRS spent $4.1 million in taxpayer money on a training conference in California that featured luxury rooms and gift giveaways.

The report – written by the same watchdog that blew the whistle on the agency's targeting of conservative groups -- showed that the IRS held 225 employee conferences from 2010 through 2012, at a total cost of $50 million.

“In light of these reports, it appears that the response provided by Treasury was inaccurate and incomplete,” Coburn wrote in his June 3 letter. “It did not include any information regarding the Internal Revenue Service, while we now understand IRS spent $50 million on conferences during this time period.”

Coburn says he’s also forwarding the disclosure discrepancies to the Treasury Inspector General for Tax Administration.

Asked about the claims, a Treasury spokesperson said: "We have received the Senator's letter and will respond to him directly."

Tuesday’s report also highlighted a 2010 conference in Anaheim, Calif., where 132 IRS officials received room upgrades. One official stayed five nights in a room that regularly goes for $3,500 a night, the IG report said, and another stayed four nights in a room that regularly goes for $1,499 a night.

The agency paid a flat daily fee of $135 per hotel room, the report said, but the upgrades were part of a package deal that added to the overall cost of the conference. Without the upgrades, the IRS could have negotiated a lower room rate, as required by agency procedures.

The inspector general's report was surfacing as the IRS came under fire again in connection with its targeting of conservative groups during the 2010 and 2012 elections.

On Capitol Hill on Tuesday, Tea Party targets testified about the additional scrutiny they received at the hands of IRS agents.

Fox News' Mike Emanuel and The Associated Press contributed to this report.