Russia landed troops outside the Crimean peninsula and onto the Ukrainian mainland for the first time on Saturday, as the Kremlin said it had received calls to "protect peaceful civilians" in eastern Ukraine and is considering the requests.

Crimea's breakaway government confirmed the incursion, saying its so-called self-defense forces, which are largely considered to be Russian troops, had seized a natural-gas distribution station in Ukraine's Kherson region near Crimea to prevent a cutoff of gas to the peninsula,...