WASHINGTON -- Veterans' health care will be dramatically affected if Congress finishes out the week without funding a program that permits many veterans to receive private-sector medical care, Department of Veterans Affairs Secretary David Shulkin warned Tuesday.

The Veterans Choice program will be depleted of funds within three to five weeks, Shulkin wrote in a notice to Sen. Johnny Isakson, R-Ga., chairman of the Senate Veterans' Affairs Committee. Without more money, the VA will soon stop referring patients to their private-sector doctors, he said.

Shulkin warned it could lead to diminished access to health care for veterans. The Choice program was established in 2014 to improve VA wait times and allow veterans to receive treatment closer to their homes. Since it was implemented, 1.9 million veterans have used the program at some point, and about one-third of VA medical appointments are completed in the private sector, according to the VA.

"VA's other community care programs will not have the capacity to handle all of the patients who will transition from [the Choice program], and the wait time for appointments at VA facilities will rise if large numbers of veterans return to VA to seek care," Shulkin wrote. "Taking these actions would have a number of negative consequences, including decreased access to care, damaged community partnerships and interrupted care continuity for veterans."

The VA was in the same position just four months ago. The Choice program nearly ran out of money in mid-August before Congress passed a bill providing $2.1 billion in emergency funding to keep it going.

In the time since, lawmakers have negotiated legislation to reform the Choice program. Three bills now exist in Congress to provide temporary funding for Choice and create a new system for private-sector care, but lawmakers haven't come to an agreement on one of them.

In the past few months, Shulkin shared his own proposal for overhauling community care programs and has urged Congress repeatedly to approve reform legislation before the end of the year.

The House is expected to recess Thursday, followed by the Senate on Friday.

In response to the letter, Isakson issued a statement Tuesday asking Senate leadership to schedule a vote on the Caring for Our Veterans Act -- a Choice reform bill that his committee sent to the Senate floor Nov. 29. The legislation, totaling $54 billion, would provide $3 billion to the Choice program and phase it out after one year.

"It is critical that we pass this bipartisan legislation before the end of the year to ensure veterans continue to have access to efficient, timely and quality health care," Isakson said in the written statement. "I urge the Senate to quickly pass this bipartisan legislation without delay."

In a statement last week, Shulkin pleaded with Congress to pass another emergency funding bill for the Choice program before the end of the year, and then come back in 2018 to discuss major reforms. With only days remaining before the holiday recess, that option was looking more likely Tuesday.

Shulkin wrote the VA spends between $200 million to $400 million each month on appointments made through the Choice program. As of Dec. 7, there was $490 million remaining, but much of that had already been spent in December.