The Obama administration is locked in a sharp new debate over whether to deploy American military forces to establish no-fly zones and safe havens in Syria to protect civilians caught in its grinding civil war.

The White House remains deeply skeptical about the idea, but the growing refugee crisis in Europe and Russia’s military intervention in Syria have increased pressure on President Obama to take more forceful action. Secretary of State John Kerry and others renewed their push at a tense White House meeting on Monday to use air power to shield Syrians from the fighting, officials said.

But at the same meeting, which included Defense Secretary Ashton B. Carter, a Pentagon report presented sobering estimates of the extensive military resources required to enforce such zones, leaving many at the table dubious about the wisdom of taking action. Russia’s own military operations in Syria raise the risk of an inadvertent clash if Americans try to block off certain parts of the country. […]