The U.S. fight against Islamic State militants risks becoming "another slow, grinding failure" like the Vietnam War, the Senate Armed Services Committee chairman warned Tuesday.

Sen. John McCain, R-Ariz., has been a frequent critic of the White House approach in the Middle East, but invoked his own past as a Vietnam War pilot in his latest rebuke. In a letter to Defense Secretary Ash Carter, McCain compared the current strategy to "the failed policy of gradual escalation" in that previous war.

"My conversations with military commanders both on the ground and in the Pentagon have led me to the disturbing, yet unavoidable conclusion that they have been reduced from considering what it will take to win to what they will be allowed to do by this administration," he wrote.

"And it will be the men and women serving in our military and our national security that will pay the price. This is unacceptable."

The comments mirror pointed attacks on the president that McCain has mentioned at recent committee hearings and public speeches.

In the letter, McCain also demands answers from Pentagon leaders on a host of issues related to the ongoing fight in the region, including a total of U.S. personnel in the region "including personnel that are not accounted for under the force management level numbers" and whether military leaders plan for similar deployments through Africa as the Islamic State group, also known as ISIS or ISIL, establishes new strongholds there.

Currently fewer than 4,000 American ground troops are deployed in support of Operation Inherent Resolve in Iraq, but lawmakers have expressed concerns with how those totals are calculated and whether some troops traveling into the region for short periods are counted.

President Obama has repeatedly promised not to put troops in the region to engage in direct fighting with ISIS militants, but has moved small teams of special operations forces to target specific ISIS leadership cells and work closely with friendly units in Iraq and Syria.

McCain said a more accurate accounting of the military's footprint there is needed to "provide the necessary resources and support our war fighters need to achieve their missions and return home safely."

During the last war in Iraq, Democrats on Capitol Hill frequently invoked the Vietnam War comparison when pressing then-President George W. Bush for an exit strategy from the region. McCain has argued that Obama's moves withdrawing troops from the country early in his presidency led to the rise of ISIS in the region.

Leo Shane III covers Congress, Veterans Affairs and the White House for Military Times. He can be reached at lshane@militarytimes.com.