Senior Republican lawmaker John McCain warned Tuesday that the United States-led coalition is headed for "slow, grinding failure" unless it scales up the fight against the Islamic State (ISIS) group in Syria and Iraq.
"I bore witness to the failed policy of gradual escalation that ultimately led to our nation's defeat in the Vietnam War," Senator McCain, a Navy veteran who heads the Senate Armed Services Committee and ran against Barack Obama in 2008, wrote in a letter to Defense Secretary Ash Carter.
"This administration's grudging incrementalism in the war against the Islamic State risks another slow, grinding failure for our nation," he warned.
McCain said military commanders had told him they have been hamstrung in their pursuit of ISIS.
"They have been reduced from considering what it will take to win to what they will be allowed to do by this administration.... This is unacceptable," McCain said.
Following the drawn-out war in Iraq, President Obama has been reluctant to commit large numbers of ground troops to anti-ISIS efforts in Syria and Iraq, preferring to partner with local forces he argues should lead the fight for their homelands.
The military also has procedures to minimize civilian deaths, prompting critics to say restrictions are too tight.
McCain called on Carter to provide detailed information on U.S. troop numbers in Iraq and Syria, and whether the Pentagon envisions sending more troops to Africa to deal with the IS group as it "metastasizes" across the continent.
The senator from Arizona has been a vocal critic of Obama’s foreign policy and his strategy against ISIS.
A year ago McCain urged the deployment of international ground forces to combat terrorists in Syria and Iraq.
The defense secretary was to meet with Obama in the White House later Tuesday along with a slew of the Pentagon's top brass to discuss defense strategies and priorities.
Speaking at the Center for Strategic and International Studies, a Washington think tank, Carter said the Pentagon is doing all it can to quicken the anti-ISIS fight, which began in August 2014.
The campaign has dealt some important defeats to the group, but the jihadists still control huge areas, including the key Syrian city of Raqa and Mosul in Iraq.
"We've got to get these guys beaten, and as soon as possible," Carter said, adding, "We are looking for every opportunity we can take to do that.... We are doing more every day and we are looking for opportunities to do yet more because we need to get this over with. I am confident we will defeat ISIL, I have no question in my mind about it."
AFP contributed to this report.