"Secretary Clinton's always quick for the military intervention," O'Malley said on CNN's State of the Union, hitting on an issue that is almost certain to come up at Tuesday night's Democratic debate in Las Vegas.
"No fly zones sound attractive, but no fly zones also have to be enforced," he added.
O'Malley, who warned of an escalating conflict with Russia of "Cold War proportions," gained attention this year by being the first to call on the Obama administration to allow 65,000 refugees fleeing violence in Syria to resettle in the United States -- an idea that has since been embraced by other campaigns.
But the former Maryland governor has generally been vague on his foreign policy in Syria and elsewhere, often casting those issues in domestic debates over climate change or the economy. His words Sunday offer a decidedly more dovish approach on Syria than Clinton has presented.
Clinton, President Barack Obama's former Secretary of State, said earlier this month that she would advocate for a no fly zone to "try to stop the carnage on the ground." The Obama administration has been cool to that idea.
Vermont Sen. Bernie Sanders, the leading challenger to Clinton, also opposes a no fly zone.
"There's many fights in this world," O'Malley said. "Not every fight is our fight."
Consistent with his approach in recent weeks, O'Malley offered few clues to his debate strategy or preparation. Analysts say that O'Malley, who is polling in the single digits in early voting states after months of campaigning, needs a breakthrough moment on Tuesday.
"For the vast majority of Americans who are searching for a new leader...this race is really just beginning for the Democratic Party," O'Malley said Sunday.
The former two-term governor reiterated his call for more Democratic debates -- a position he has sounded for weeks. He has been critical of his own party for sanctioning only six debates.
Democratic National Committee chairwoman Debbie Wasserman Schultz, who followed O'Malley on CNN Sunday, declined to address the issue directly: "We're excited about the first debate coming up on Tuesday," she said.
Wasserman Schultz has been working to promote the debate among party faithful. The DNC sent an email to members last week asking them to use social media sites like Facebook to organize watch parties and to "identify local voices" who can offer reaction to news outlets.
"We don't want what happens in Vegas to stay in Vegas," Wasserman Schultz wrote in the email.