Defense Secretary Jim Mattis said he will head to Capitol Hill on Wednesday, following his order to increase the number of U.S. troops in Afghanistan and amid growing concerns Congress is set to punt on the annual defense budget.

New Afghanistan expenses and a delayed budget could create hardships for Pentagon finances this fall and Mattis said he will meet with lawmakers to brief them on troop numbers and work out a plan to fund the military beginning in October.

Congress appears poised to pass a continuing budget resolution, or CR, at the end of this month instead of an annual defense budget, temporarily locking in current funding levels that do not include the increased operations in Afghanistan. Sen. John McCain, R-Ariz., has warned a CR could cut billions from planned defense spending and cause problems for the Pentagon.

"We will work with [Congress]. We've already got meetings scheduled to sit down with them and talk with them about the way forward," Mattis said this week.

Mattis said he signed orders to begin sending additional troops to join the existing 11,000 deployed to Afghanistan and that he plans to send more. The defense secretary declined to provide troop numbers until he briefs Congress, but he is expected to ultimately approve up to about 4,000.

In November, the Obama administration requested nearly $12 billion in additional funding from Congress to keep additional troops in Afghanistan and for the fight against the Islamic State in Iraq and Syria.

Mattis and the Pentagon may look to request their own supplemental Afghanistan spending as part of any CR legislation considered by Congress this month, said Katherine Blakeley, a research fellow at the Center for Strategic and Budgetary Assessments.

The temporary budget measure would have to be approved by lawmakers by Oct. 1 when the current federal budget expires or a government shutdown could occur. Many in Congress are loathe to close the government, despite warnings by President Trump, which could be a powerful motivator to pass a CR.

"If I saw that I could get this [Afghanistan] element funded now, I would want to seize that opportunity," Blakeley said.

On the spending side, Mattis will find a welcome audience with McCain. The Arizona senator, along with Ranking Member Sen. Jack Reed, D-R.I., asked Mattis this week to list all the ways a continuing resolution would harm the Pentagon's budget.

"In practice, this will result in billions of dollars in cuts to the defense budget from last year's level — cuts that the Department of Defense can ill afford at a time of diminished readiness, strained modernization, and increasing operations," McCain and Reed wrote in the letter.