Ukrainian officials have said for months that they would not cede territory occupied by Russia in any peace settlement. Now, as Ukraine contemplates an accelerated timetable for negotiations pushed by President-elect Donald J. Trump, it is putting at least as much importance on obtaining security guarantees as on where an eventual cease-fire line might fall.

With Ukrainian forces steadily losing ground in the east, two senior officials said that defending Ukraine’s interests in potential talks would hinge not on territorial boundaries, which are likely to be determined by the fighting, but on what assurances are in place to make a cease-fire hold.

“Talks should be based on guarantees,” said Roman Kostenko, the chairman of the Ukrainian Parliament’s Defense and Intelligence Committee. “For Ukraine, nothing is more important.”

A senior Ukrainian official, speaking on condition of anonymity to discuss sensitive negotiations, was more direct. “The territorial question is extremely important, but it’s still the second question,” the official said, “The first question is security guarantees.”

Ukraine sets its borders based on its 1991 declaration of independence. Russia has since gained control of about 20 percent of Ukrainian land, but Kyiv would not formally renounce its claim over any territory under Russian occupation, Mr. Kostenko said.

That appears to be the approach Ukraine is taking to justify any possible deal in which Russia would retain control of Ukrainian land. In October, President Volodymyr Zelensky, discussing a cease-fire, said “Everyone understands that no matter what path we take, legally no one will recognize the occupied territories as belonging to other countries.”