SIMFEROPOL, Ukraine — Tensions mounted on the eve of a secession referendum here in Crimea as helicopter-borne Russian forces made a provocative incursion just outside the peninsula’s regional border to seize a natural gas terminal, while American and European officials prepared sanctions to impose on Moscow as early as Monday.

The military operation by at least 80 troops landing on a slender sand bar just across from Crimea’s northeast border seemed part of a broader effort to strengthen control over the peninsula before a referendum Sunday on whether its majority Russian-speaking population wants to demand greater autonomy from Ukraine or break away completely and join Russia. Whatever its tactical goals, the seizure of the terminal sent a defiant message to the United States and Europe and underscored that a diplomatic resolution to Russia’s recent takeover of Crimea remains elusive.

The raid came as American and European diplomats at the United Nations pushed for a vote on a resolution declaring the Sunday referendum illegal, essentially forcing Russia to veto the measure. In the end, Russia cast the only vote against it; even China, its traditional ally on the Council, did not vote with Moscow but abstained, an indication of its unease with Russia’s violation of another country’s sovereignty. Western diplomats hoped the result would reinforce Russia’s growing international isolation over Ukraine.