Secretary of Defense Ashton Carter warned that a plan for humanitarian intervention four senators want the president to consider would amount to a “major combat mission.”

Carter told Sen. Dick Durbin (D-Ill.) on Wednesday at a senate appropriations subcommittee hearing that the creation of safe zones within Syria is something that would be “contested” by both the Syrian government and fundamentalist insurgent groups like the Islamic State (ISIL).

“We would need to fight to create such a space and then fight to keep such a space, and that’s why it’s a difficult thing to contemplate,” Carter said.

He noted, in his back-and-forth with Durbin, that any US operation would probably not be supported by countries bordering Syria “and therefore something we would do ourselves.”

At one point, he stressed that “we have thought this through.”

Outgoing Joint Chiefs of Staff chair, Gen. Martin Dempsey, also remarked that “we’ve been planning for such a contingency for some time,” and noted that the Turkish military has taken part in brainstorming. He also called the blueprints “practical, militarily” but lamented “opportunity costs” and the fact that lawmakers are asking the Pentagon to be subject to both additional responsibilities and cost-cutting mandated by the Budget Control Act of 2011.

“That is to say, resources we have deployed elsewhere would have to be repositioned,” Dempsey commented.

In April, Sens. Durbin, Tim Kaine (D-Va.), Lindsey Graham (R-S.C.) and John McCain (R-Ariz.) wrote to President Obama, asking him to deepen US intervention in Syria to forge “one or more safe zones, ‘with necessary enforceable mechanisms,’” according to Reuters .

Durbin noted Wednesday that he signed onto the letter “with some reluctance, realizing it would require a commitment by the United States.”

The leading liberal senator also said that he hoped the plan would be executed in conjunction with the United Nations or some other kind of coalition.

While Carter was less enthusiastic than Durbin about direct intervention in this case, he touted US efforts at advancing regime change through the brutal civil war being waged–now, largely, between Syrian government forces and fundamentalist insurgents like ISIL.

“As you know, we’re trying to create a third force to combat those two and to create an environment in which the Syrian people can leave in peace, which they deserve,” he said.