The inspection teams have also coordinated the breakdown of thousands of metric tons of unused chemical munitions as part of the ongoing effort to eliminate those stockpiles, according to OPCW officials.
But as inspections continue and investigators secure and destroy more and more of Assad's stockpile, the mission may become too unwieldy for the OPCW teams and require NATO support.
"I think it would probably be something we would assume would occur if we can stay on track and make progress . . . . [and] that other nations would be asked for help," Hagel said.
Increasing security threats to the inspection teams in Syria also could prompt military support from NATO.
Inspectors have been repeatedly targeted with roadside bombs, the same weapon of choice being used against American forces in Afghanistan, while attempting to dismantle Syria's chemical weapon stockpiles.
Earlier this month, a mortar attack was launched against the Damascus hotel where the inspection teams were staying, according to recent reports.
The escalating violence directed at the international inspectors has created a "cause for concern" among the teams, OPCW spokesman Michael Luhan told the BBC.
That concern has forced inspectors to temporarily abandon disarmament work at certain chemical weapon sites, Luhan said in an interview Thursday.
That said, Hagel reiterated that even if NATO forces were sent in to back up the OPCW inspectors, U.S. troops would not be part of that operation.
"There are no plans to have any U.S. forces in any way in Syria," he said.
However, U.S. military commanders in the Mideast have drafted up a slate of options to address possible security challenges facing Washington and allied efforts to disarm Syria.
Central Command submitted their recommendations to U.S. diplomats, a State Department official said in September before the inspections began.
In July, Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, in July told leaders of the Senate Armed Forces Committee that a significant deployment of ground troops would be needed to secure Syria’s chemical weapon sites if an operation of that kind were ordered.
"Thousands of special operations forces and other ground forces would be needed to assault and secure critical sites" in the country, Dempsey wrote in a letter.
Pentagon estimates at the time reportedly said 70,000 troops would be needed to lockdown Assad's chemical weapons program.