Prosecutors in Ukraine are investigating whether a member of Parliament committed treason by working with two associates of President Trump’s to promote a plan for settling Ukraine’s conflicts with Russia.

In a court filing on Tuesday, prosecutors accused the lawmaker, Andrii V. Artemenko, of conspiring with Russia to commit “subversive acts against Ukraine,” in particular by advancing a proposal that could “legitimize the temporary occupation” of the Crimean peninsula. Russia forcibly annexed the peninsula in 2014, a step that Ukraine, the United States and other governments have refused to recognize; Mr. Artemenko said his proposal would allow Ukraine to formally cede control of the territory to Russia, at least temporarily.

Yuriy Lutsenko, Ukraine’s prosecutor general, posted a copy of the court filing to his Facebook page on Tuesday with the statement “Ukraine’s integrity is above all else.”

Mr. Artemenko’s plan, reported on Sunday by The New York Times, outlines a series of steps meant to bring to an end the rebellion by Russian-backed separatists in eastern Ukraine, and to resolve the dispute over Crimea by allowing voters to decide whether to lease the peninsula to Russia for 50 or 100 years.