Washington (CNN) -- A sharp disagreement over the future treatment of undocumented workers on America's farms is holding up progress on the comprehensive immigration reform bill being considered by the Senate's so-called "Gang of Eight."
The two key sticking points are wages and the number of visas to be granted to undocumented farm workers, two sources close to the talks confirmed Friday.
"It's the one major unresolved issue," one of the sources told CNN.
The sources were unable to provide specifics in terms of the number of visas or wage levels under consideration. But, generally speaking, agricultural businesses have an interest in more visas and lower wages. Labor leaders, in contrast, typically support fewer visas and higher wages.
Aside from the agricultural dispute, both sources stressed that the sheer complexity of the major reform bill could slow the timetable for its public unveiling. Senators are hoping to release the bill next week, but a significant amount of time is required for staffers from multiple offices to work their way through the lengthy proposal.
The Democratic-controlled Senate and the GOP-controlled House each are working on immigration reform plans. Both chambers are expected to advance their respective blueprints some point soon after Congress returns next week from its spring break.
The agricultural dispute is just one component of a plan that would provide the first major overhaul of America's immigration laws since 1986.
Among other things, the Senate bill aims to provide a path to citizenship for an estimated 11 million undocumented residents. It would also create a new system for assessing the state of border security.
Members of the Senate's "Gang of Eight" are Sen. Lindsey Graham, R-South Carolina; Sen. Marco Rubio, R-Florida; Sen. John McCain, R-Arizona; Sen. Jeff Flake, R-Arizona; Sen. Robert Menendez, D-New Jersey; Sen. Dick Durbin, D-Illinois; Sen. Michael Bennet, D-Colorado, and Sen. Chuck Schumer, D-New York.
A separate group of four senators -- Rubio, Bennet, Utah Republican Orrin Hatch and California Democrat Dianne Feinstein -- is trying to resolve the agricultural dispute.