DONETSK, Ukraine — After Ukraine’s new president, Petro O. Poroshenko, told reporters in Kiev on Wednesday that he might soon order a temporary, unilateral cease-fire as part of a broader 14-point peace plan, it took all of several seconds for pro-Russian militants to rule it out.

“I am a condemned man,” said a stick-thin fighter who, like many others here, identified himself only by an alias, Tarik, for security reasons. Sipping tea in the gloom of the lobby of Donetsk’s rebel-occupied administration building on Wednesday afternoon, he patted the magazine of the automatic rifle slung across his chest.

Any cease-fire would certainly be violated by the Ukrainian Army, he said, adding that he and other pro-Russian separatists would be arrested the minute the government had the opportunity.

“What peace can they possibly offer me?” he asked. “If they want peace, then they can leave.”

Tarik and a dozen other rank-and-file fighters here reacted to Mr. Poroshenko’s proposal with a dark, belligerent skepticism. Most rejected the idea of disarming until a patchwork of amorphous conditions were met, suggesting that a truce would be awfully difficult to achieve.