WARSAW — Russian President Vladimir Putin’s massive military offensive in Ukraine has prompted European officials on the edge of the war zone to contemplate the prospect that he might use nuclear weapons to achieve his objectives — or even consider an attack on a NATO-allied state.

“Nuclear war is impossible to be won. … This is the rule of warfare, that, actually, it’s the last resort,” a senior European official said. “But, unfortunately, Russians actually somehow behave too lightly about it. And that causes the biggest trouble for us.”