When Republicans release their long-awaited tax reform plan the week of Sept. 25, the document will "absolutely" include specifics on matters such as tax rates and which tax breaks they will seek to eliminate, Treasury Secretary Steven Mnuchin said Thursday.

"You'll have extensive details on all of that," Mnuchin said at an event hosted by Politico in Washington.

Mnuchin and the White House have been working for months with congressional Republican leadership to find a tax plan that would unite the GOP. To date, though, all they have produced is a few vague paragraphs that they released in July, containing no specifics.

The "Big Six," as the administration and congressional leaders negotiating the plan are called, are facing increasing pressure to outline a tax reform plan. Conservative House members have demanded that the leadership sketch out some details before the chamber approves a budget.

This week, they said a document would be released later in the week of Sept. 25, allowing the committees to then begin writing legislation.

Yet some conservatives have expressed skepticism that the plan will contain the level of detail they need to be confident in begining the legislative process, starting with adopting a budget resolution.

"I'm not going to go through the details today," Mnuchin said, but "we have a unified plan."

Earlier, on the same stage, House Ways and Means Committee Chairman Kevin Brady downplayed the level of specificity in the document.

A third member of the Big Six, Senate Finance Committee Chairman Orrin Hatch, also sought Thursday to tamper expectations about the forthcoming document.

The guidance "will not dictate the direction we take in this committee," the Utah Republican said at a hearing of the Finance Committee. "Any forthcoming documents may be viewed as guidance or potential signposts for drafting legislation."

"We are not anyone's rubber stamp," Hatch added.

• This post has been updated to reflect Mnuchin's and Hatch's comments.